summaryrefslogtreecommitdiffstats
path: root/doc/src/sgml/ref/select.sgml
blob: 16bbab52c3e6fe40347bbb029cb994cfb1429a3a (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
<!--
doc/src/sgml/ref/select.sgml
PostgreSQL documentation
-->

<refentry id="sql-select">
 <indexterm zone="sql-select">
  <primary>SELECT</primary>
 </indexterm>

 <indexterm zone="sql-select">
  <primary>TABLE command</primary>
 </indexterm>

 <indexterm zone="sql-select">
  <primary>WITH</primary>
  <secondary>in SELECT</secondary>
 </indexterm>

 <refmeta>
  <refentrytitle>SELECT</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>SELECT</refname>
  <refname>TABLE</refname>
  <refname>WITH</refname>
  <refpurpose>retrieve rows from a table or view</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
[ WITH [ RECURSIVE ] <replaceable class="parameter">with_query</replaceable> [, ...] ]
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replaceable> [, ...] ) ] ]
    [ * | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
    [ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
    [ WHERE <replaceable class="parameter">condition</replaceable> ]
    [ GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...] ]
    [ HAVING <replaceable class="parameter">condition</replaceable> ]
    [ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...] ]
    [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] <replaceable class="parameter">select</replaceable> ]
    [ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...] ]
    [ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="parameter">start</replaceable> [ ROW | ROWS ] ]
    [ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } { ONLY | WITH TIES } ]
    [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]

<phrase>where <replaceable class="parameter">from_item</replaceable> can be one of:</phrase>

    [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
                [ TABLESAMPLE <replaceable class="parameter">sampling_method</replaceable> ( <replaceable class="parameter">argument</replaceable> [, ...] ) [ REPEATABLE ( <replaceable class="parameter">seed</replaceable> ) ] ]
    [ LATERAL ] ( <replaceable class="parameter">select</replaceable> ) [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ]
    <replaceable class="parameter">with_query_name</replaceable> [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
    [ LATERAL ] <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] )
                [ WITH ORDINALITY ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
    [ LATERAL ] <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) [ AS ] <replaceable class="parameter">alias</replaceable> ( <replaceable class="parameter">column_definition</replaceable> [, ...] )
    [ LATERAL ] <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) AS ( <replaceable class="parameter">column_definition</replaceable> [, ...] )
    [ LATERAL ] ROWS FROM( <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) [ AS ( <replaceable class="parameter">column_definition</replaceable> [, ...] ) ] [, ...] )
                [ WITH ORDINALITY ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
    <replaceable class="parameter">from_item</replaceable> [ NATURAL ] <replaceable class="parameter">join_type</replaceable> <replaceable class="parameter">from_item</replaceable> [ ON <replaceable class="parameter">join_condition</replaceable> | USING ( <replaceable class="parameter">join_column</replaceable> [, ...] ) [ AS <replaceable class="parameter">join_using_alias</replaceable> ] ]

<phrase>and <replaceable class="parameter">grouping_element</replaceable> can be one of:</phrase>

    ( )
    <replaceable class="parameter">expression</replaceable>
    ( <replaceable class="parameter">expression</replaceable> [, ...] )
    ROLLUP ( { <replaceable class="parameter">expression</replaceable> | ( <replaceable class="parameter">expression</replaceable> [, ...] ) } [, ...] )
    CUBE ( { <replaceable class="parameter">expression</replaceable> | ( <replaceable class="parameter">expression</replaceable> [, ...] ) } [, ...] )
    GROUPING SETS ( <replaceable class="parameter">grouping_element</replaceable> [, ...] )

<phrase>and <replaceable class="parameter">with_query</replaceable> is:</phrase>

    <replaceable class="parameter">with_query_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ] AS [ [ NOT ] MATERIALIZED ] ( <replaceable class="parameter">select</replaceable> | <replaceable class="parameter">values</replaceable> | <replaceable class="parameter">insert</replaceable> | <replaceable class="parameter">update</replaceable> | <replaceable class="parameter">delete</replaceable> )
        [ SEARCH { BREADTH | DEPTH } FIRST BY <replaceable>column_name</replaceable> [, ...] SET <replaceable>search_seq_col_name</replaceable> ]
        [ CYCLE <replaceable>column_name</replaceable> [, ...] SET <replaceable>cycle_mark_col_name</replaceable> [ TO <replaceable>cycle_mark_value</replaceable> DEFAULT <replaceable>cycle_mark_default</replaceable> ] USING <replaceable>cycle_path_col_name</replaceable> ]

TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
</synopsis>

 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>SELECT</command> retrieves rows from zero or more tables.
   The general processing of <command>SELECT</command> is as follows:

   <orderedlist>
    <listitem>
     <para>
      All queries in the <literal>WITH</literal> list are computed.
      These effectively serve as temporary tables that can be referenced
      in the <literal>FROM</literal> list.  A <literal>WITH</literal> query
      that is referenced more than once in <literal>FROM</literal> is
      computed only once,
      unless specified otherwise with <literal>NOT MATERIALIZED</literal>.
      (See <xref linkend="sql-with"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      All elements in the <literal>FROM</literal> list are computed.
      (Each element in the <literal>FROM</literal> list is a real or
      virtual table.)  If more than one element is specified in the
      <literal>FROM</literal> list, they are cross-joined together.
      (See <xref linkend="sql-from"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      If the <literal>WHERE</literal> clause is specified, all rows
      that do not satisfy the condition are eliminated from the
      output.  (See <xref linkend="sql-where"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      If the <literal>GROUP BY</literal> clause is specified,
      or if there are aggregate function calls, the
      output is combined into groups of rows that match on one or more
      values, and the results of aggregate functions are computed.
      If the <literal>HAVING</literal> clause is present, it
      eliminates groups that do not satisfy the given condition.  (See
      <xref linkend="sql-groupby"/> and
      <xref linkend="sql-having"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      The actual output rows are computed using the
      <command>SELECT</command> output expressions for each selected
      row or row group.  (See <xref linkend="sql-select-list"/> below.)
     </para>
    </listitem>

    <listitem>
     <para><literal>SELECT DISTINCT</literal> eliminates duplicate rows from the
      result.  <literal>SELECT DISTINCT ON</literal> eliminates rows that
      match on all the specified expressions.  <literal>SELECT ALL</literal>
      (the default) will return all candidate rows, including
      duplicates.  (See <xref linkend="sql-distinct"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      Using the operators <literal>UNION</literal>,
      <literal>INTERSECT</literal>, and <literal>EXCEPT</literal>, the
      output of more than one <command>SELECT</command> statement can
      be combined to form a single result set.  The
      <literal>UNION</literal> operator returns all rows that are in
      one or both of the result sets.  The
      <literal>INTERSECT</literal> operator returns all rows that are
      strictly in both result sets.  The <literal>EXCEPT</literal>
      operator returns the rows that are in the first result set but
      not in the second.  In all three cases, duplicate rows are
      eliminated unless <literal>ALL</literal> is specified.  The noise
      word <literal>DISTINCT</literal> can be added to explicitly specify
      eliminating duplicate rows.  Notice that <literal>DISTINCT</literal> is
      the default behavior here, even though <literal>ALL</literal> is
      the default for <command>SELECT</command> itself.  (See
      <xref linkend="sql-union"/>, <xref linkend="sql-intersect"/>, and
      <xref linkend="sql-except"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      If the <literal>ORDER BY</literal> clause is specified, the
      returned rows are sorted in the specified order.  If
      <literal>ORDER BY</literal> is not given, the rows are returned
      in whatever order the system finds fastest to produce.  (See
      <xref linkend="sql-orderby"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      If the <literal>LIMIT</literal> (or <literal>FETCH FIRST</literal>) or <literal>OFFSET</literal>
      clause is specified, the <command>SELECT</command> statement
      only returns a subset of the result rows. (See <xref
      linkend="sql-limit"/> below.)
     </para>
    </listitem>

    <listitem>
     <para>
      If <literal>FOR UPDATE</literal>, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR SHARE</literal>
      or <literal>FOR KEY SHARE</literal>
      is specified, the
      <command>SELECT</command> statement locks the selected rows
      against concurrent updates.  (See <xref linkend="sql-for-update-share"/>
      below.)
     </para>
    </listitem>
   </orderedlist>
  </para>

  <para>
   You must have <literal>SELECT</literal> privilege on each column used
   in a <command>SELECT</command> command.  The use of <literal>FOR NO KEY UPDATE</literal>,
   <literal>FOR UPDATE</literal>,
   <literal>FOR SHARE</literal> or <literal>FOR KEY SHARE</literal> requires
   <literal>UPDATE</literal> privilege as well (for at least one column
   of each table so selected).
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <refsect2 id="sql-with" xreflabel="WITH Clause">
   <title><literal>WITH</literal> Clause</title>

   <para>
    The <literal>WITH</literal> clause allows you to specify one or more
    subqueries that can be referenced by name in the primary query.
    The subqueries effectively act as temporary tables or views
    for the duration of the primary query.
    Each subquery can be a <command>SELECT</command>, <command>TABLE</command>, <command>VALUES</command>,
    <command>INSERT</command>, <command>UPDATE</command> or
    <command>DELETE</command> statement.
    When writing a data-modifying statement (<command>INSERT</command>,
    <command>UPDATE</command> or <command>DELETE</command>) in
    <literal>WITH</literal>, it is usual to include a <literal>RETURNING</literal> clause.
    It is the output of <literal>RETURNING</literal>, <emphasis>not</emphasis> the underlying
    table that the statement modifies, that forms the temporary table that is
    read by the primary query.  If <literal>RETURNING</literal> is omitted, the
    statement is still executed, but it produces no output so it cannot be
    referenced as a table by the primary query.
   </para>

   <para>
    A name (without schema qualification) must be specified for each
    <literal>WITH</literal> query.  Optionally, a list of column names
    can be specified; if this is omitted,
    the column names are inferred from the subquery.
   </para>

   <para>
    If <literal>RECURSIVE</literal> is specified, it allows a
    <command>SELECT</command> subquery to reference itself by name.  Such a
    subquery must have the form
<synopsis>
<replaceable class="parameter">non_recursive_term</replaceable> UNION [ ALL | DISTINCT ] <replaceable class="parameter">recursive_term</replaceable>
</synopsis>
    where the recursive self-reference must appear on the right-hand
    side of the <literal>UNION</literal>.  Only one recursive self-reference
    is permitted per query.  Recursive data-modifying statements are not
    supported, but you can use the results of a recursive
    <command>SELECT</command> query in
    a data-modifying statement.  See <xref linkend="queries-with"/> for
    an example.
   </para>

   <para>
    Another effect of <literal>RECURSIVE</literal> is that
    <literal>WITH</literal> queries need not be ordered: a query
    can reference another one that is later in the list.  (However,
    circular references, or mutual recursion, are not implemented.)
    Without <literal>RECURSIVE</literal>, <literal>WITH</literal> queries
    can only reference sibling <literal>WITH</literal> queries
    that are earlier in the <literal>WITH</literal> list.
   </para>

   <para>
    When there are multiple queries in the <literal>WITH</literal>
    clause, <literal>RECURSIVE</literal> should be written only once,
    immediately after <literal>WITH</literal>.  It applies to all queries
    in the <literal>WITH</literal> clause, though it has no effect on
    queries that do not use recursion or forward references.
   </para>

   <para>
    The optional <literal>SEARCH</literal> clause computes a <firstterm>search
    sequence column</firstterm> that can be used for ordering the results of a
    recursive query in either breadth-first or depth-first order.  The
    supplied column name list specifies the row key that is to be used for
    keeping track of visited rows.  A column named
    <replaceable>search_seq_col_name</replaceable> will be added to the result
    column list of the <literal>WITH</literal> query.  This column can be
    ordered by in the outer query to achieve the respective ordering.  See
    <xref linkend="queries-with-search"/> for examples.
   </para>

   <para>
    The optional <literal>CYCLE</literal> clause is used to detect cycles in
    recursive queries.  The supplied column name list specifies the row key
    that is to be used for keeping track of visited rows.  A column named
    <replaceable>cycle_mark_col_name</replaceable> will be added to the result
    column list of the <literal>WITH</literal> query.  This column will be set
    to <replaceable>cycle_mark_value</replaceable> when a cycle has been
    detected, else to <replaceable>cycle_mark_default</replaceable>.
    Furthermore, processing of the recursive union will stop when a cycle has
    been detected.  <replaceable>cycle_mark_value</replaceable> and
    <replaceable>cycle_mark_default</replaceable> must be constants and they
    must be coercible to a common data type, and the data type must have an
    inequality operator.  (The SQL standard requires that they be Boolean
    constants or character strings, but PostgreSQL does not require that.)  By
    default, <literal>TRUE</literal> and <literal>FALSE</literal> (of type
    <type>boolean</type>) are used.  Furthermore, a column
    named <replaceable>cycle_path_col_name</replaceable> will be added to the
    result column list of the <literal>WITH</literal> query.  This column is
    used internally for tracking visited rows.  See <xref
    linkend="queries-with-cycle"/> for examples.
   </para>

   <para>
    Both the <literal>SEARCH</literal> and the <literal>CYCLE</literal> clause
    are only valid for recursive <literal>WITH</literal> queries.  The
    <replaceable>with_query</replaceable> must be a <literal>UNION</literal>
    (or <literal>UNION ALL</literal>) of two <literal>SELECT</literal> (or
    equivalent) commands (no nested <literal>UNION</literal>s).  If both
    clauses are used, the column added by the <literal>SEARCH</literal> clause
    appears before the columns added by the <literal>CYCLE</literal> clause.
   </para>

   <para>
    The primary query and the <literal>WITH</literal> queries are all
    (notionally) executed at the same time.  This implies that the effects of
    a data-modifying statement in <literal>WITH</literal> cannot be seen from
    other parts of the query, other than by reading its <literal>RETURNING</literal>
    output.  If two such data-modifying statements attempt to modify the same
    row, the results are unspecified.
   </para>

   <para>
    A key property of <literal>WITH</literal> queries is that they
    are normally evaluated only once per execution of the primary query,
    even if the primary query refers to them more than once.
    In particular, data-modifying statements are guaranteed to be
    executed once and only once, regardless of whether the primary query
    reads all or any of their output.
   </para>

   <para>
    However, a <literal>WITH</literal> query can be marked
    <literal>NOT MATERIALIZED</literal> to remove this guarantee.  In that
    case, the <literal>WITH</literal> query can be folded into the primary
    query much as though it were a simple sub-<literal>SELECT</literal> in
    the primary query's <literal>FROM</literal> clause.  This results in
    duplicate computations if the primary query refers to
    that <literal>WITH</literal> query more than once; but if each such use
    requires only a few rows of the <literal>WITH</literal> query's total
    output, <literal>NOT MATERIALIZED</literal> can provide a net savings by
    allowing the queries to be optimized jointly.
    <literal>NOT MATERIALIZED</literal> is ignored if it is attached to
    a <literal>WITH</literal> query that is recursive or is not
    side-effect-free (i.e., is not a plain <literal>SELECT</literal>
    containing no volatile functions).
   </para>

   <para>
    By default, a side-effect-free <literal>WITH</literal> query is folded
    into the primary query if it is used exactly once in the primary
    query's <literal>FROM</literal> clause.  This allows joint optimization
    of the two query levels in situations where that should be semantically
    invisible.  However, such folding can be prevented by marking the
    <literal>WITH</literal> query as <literal>MATERIALIZED</literal>.
    That might be useful, for example, if the <literal>WITH</literal> query
    is being used as an optimization fence to prevent the planner from
    choosing a bad plan.
    <productname>PostgreSQL</productname> versions before v12 never did
    such folding, so queries written for older versions might rely on
    <literal>WITH</literal> to act as an optimization fence.
   </para>

   <para>
    See <xref linkend="queries-with"/> for additional information.
   </para>
  </refsect2>

  <refsect2 id="sql-from" xreflabel="FROM Clause">
   <title><literal>FROM</literal> Clause</title>

   <para>
    The <literal>FROM</literal> clause specifies one or more source
    tables for the <command>SELECT</command>.  If multiple sources are
    specified, the result is the Cartesian product (cross join) of all
    the sources.  But usually qualification conditions are added (via
    <literal>WHERE</literal>) to restrict the returned rows to a small subset of the
    Cartesian product.
   </para>

   <para>
    The <literal>FROM</literal> clause can contain the following
    elements:

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">table_name</replaceable></term>
      <listitem>
       <para>
        The name (optionally schema-qualified) of an existing table or view.
        If <literal>ONLY</literal> is specified before the table name, only that
        table is scanned.  If <literal>ONLY</literal> is not specified, the table
        and all its descendant tables (if any) are scanned.  Optionally,
        <literal>*</literal> can be specified after the table name to explicitly
        indicate that descendant tables are included.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">alias</replaceable></term>
      <listitem>
       <para>
        A substitute name for the <literal>FROM</literal> item containing the
        alias.  An alias is used for brevity or to eliminate ambiguity
        for self-joins (where the same table is scanned multiple
        times).  When an alias is provided, it completely hides the
        actual name of the table or function; for example given
        <literal>FROM foo AS f</literal>, the remainder of the
        <command>SELECT</command> must refer to this <literal>FROM</literal>
        item as <literal>f</literal> not <literal>foo</literal>.  If an alias is
        written, a column alias list can also be written to provide
        substitute names for one or more columns of the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>TABLESAMPLE <replaceable class="parameter">sampling_method</replaceable> ( <replaceable class="parameter">argument</replaceable> [, ...] ) [ REPEATABLE ( <replaceable class="parameter">seed</replaceable> ) ]</literal></term>
      <listitem>
       <para>
        A <literal>TABLESAMPLE</literal> clause after
        a <replaceable class="parameter">table_name</replaceable> indicates that the
        specified <replaceable class="parameter">sampling_method</replaceable>
        should be used to retrieve a subset of the rows in that table.
        This sampling precedes the application of any other filters such
        as <literal>WHERE</literal> clauses.
        The standard <productname>PostgreSQL</productname> distribution
        includes two sampling methods, <literal>BERNOULLI</literal>
        and <literal>SYSTEM</literal>, and other sampling methods can be
        installed in the database via extensions.
       </para>

       <para>
        The <literal>BERNOULLI</literal> and <literal>SYSTEM</literal> sampling methods
        each accept a single <replaceable class="parameter">argument</replaceable>
        which is the fraction of the table to sample, expressed as a
        percentage between 0 and 100.  This argument can be
        any <type>real</type>-valued expression.  (Other sampling methods might
        accept more or different arguments.)  These two methods each return
        a randomly-chosen sample of the table that will contain
        approximately the specified percentage of the table's rows.
        The <literal>BERNOULLI</literal> method scans the whole table and
        selects or ignores individual rows independently with the specified
        probability.
        The <literal>SYSTEM</literal> method does block-level sampling with
        each block having the specified chance of being selected; all rows
        in each selected block are returned.
        The <literal>SYSTEM</literal> method is significantly faster than
        the <literal>BERNOULLI</literal> method when small sampling
        percentages are specified, but it may return a less-random sample of
        the table as a result of clustering effects.
       </para>

       <para>
        The optional <literal>REPEATABLE</literal> clause specifies
        a <replaceable class="parameter">seed</replaceable> number or expression to use
        for generating random numbers within the sampling method.  The seed
        value can be any non-null floating-point value.  Two queries that
        specify the same seed and <replaceable class="parameter">argument</replaceable>
        values will select the same sample of the table, if the table has
        not been changed meanwhile.  But different seed values will usually
        produce different samples.
        If <literal>REPEATABLE</literal> is not given then a new random
        sample is selected for each query, based upon a system-generated seed.
        Note that some add-on sampling methods do not
        accept <literal>REPEATABLE</literal>, and will always produce new
        samples on each use.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">select</replaceable></term>
      <listitem>
       <para>
        A sub-<command>SELECT</command> can appear in the
        <literal>FROM</literal> clause.  This acts as though its
        output were created as a temporary table for the duration of
        this single <command>SELECT</command> command.  Note that the
        sub-<command>SELECT</command> must be surrounded by
        parentheses, and an alias <emphasis>must</emphasis> be
        provided for it.  A
        <link linkend="sql-values"><command>VALUES</command></link> command
        can also be used here.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">with_query_name</replaceable></term>
      <listitem>
       <para>
        A <literal>WITH</literal> query is referenced by writing its name,
        just as though the query's name were a table name.  (In fact,
        the <literal>WITH</literal> query hides any real table of the same name
        for the purposes of the primary query.  If necessary, you can
        refer to a real table of the same name by schema-qualifying
        the table's name.)
        An alias can be provided in the same way as for a table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">function_name</replaceable></term>
      <listitem>
       <para>
        Function calls can appear in the <literal>FROM</literal>
        clause.  (This is especially useful for functions that return
        result sets, but any function can be used.)  This acts as
        though the function's output were created as a temporary table for the
        duration of this single <command>SELECT</command> command.
        If the function's result type is composite (including the case of a
        function with multiple <literal>OUT</literal> parameters), each
        attribute becomes a separate column in the implicit table.
       </para>

       <para>
        When the optional <command>WITH ORDINALITY</command> clause is added
        to the function call, an additional column of type <type>bigint</type>
        will be appended to the function's result column(s).  This column
        numbers the rows of the function's result set, starting from 1.
        By default, this column is named <literal>ordinality</literal>.
       </para>

       <para>
        An alias can be provided in the same way as for a table.
        If an alias is written, a column
        alias list can also be written to provide substitute names for
        one or more attributes of the function's composite return
        type, including the ordinality column if present.
       </para>

       <para>
        Multiple function calls can be combined into a
        single <literal>FROM</literal>-clause item by surrounding them
        with <literal>ROWS FROM( ... )</literal>.  The output of such an item is the
        concatenation of the first row from each function, then the second
        row from each function, etc.  If some of the functions produce fewer
        rows than others, null values are substituted for the missing data, so
        that the total number of rows returned is always the same as for the
        function that produced the most rows.
       </para>

      <para>
        If the function has been defined as returning the
        <type>record</type> data type, then an alias or the key word
        <literal>AS</literal> must be present, followed by a column
        definition list in the form <literal>( <replaceable
        class="parameter">column_name</replaceable> <replaceable
        class="parameter">data_type</replaceable> <optional>, ...
        </optional>)</literal>.  The column definition list must match the
        actual number and types of columns returned by the function.
       </para>

       <para>
        When using the <literal>ROWS FROM( ... )</literal> syntax, if one of the
        functions requires a column definition list, it's preferred to put
        the column definition list after the function call inside
        <literal>ROWS FROM( ... )</literal>.  A column definition list can be placed
        after the <literal>ROWS FROM( ... )</literal> construct only if there's just
        a single function and no <literal>WITH ORDINALITY</literal> clause.
       </para>

       <para>
        To use <literal>ORDINALITY</literal> together with a column definition
        list, you must use the <literal>ROWS FROM( ... )</literal> syntax and put the
        column definition list inside <literal>ROWS FROM( ... )</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">join_type</replaceable></term>
      <listitem>
       <para>
        One of
        <itemizedlist>
         <listitem>
          <para><literal>[ INNER ] JOIN</literal></para>
         </listitem>
         <listitem>
          <para><literal>LEFT [ OUTER ] JOIN</literal></para>
         </listitem>
         <listitem>
          <para><literal>RIGHT [ OUTER ] JOIN</literal></para>
         </listitem>
         <listitem>
          <para><literal>FULL [ OUTER ] JOIN</literal></para>
         </listitem>
         <listitem>
          <para><literal>CROSS JOIN</literal></para>
         </listitem>
        </itemizedlist>

        For the <literal>INNER</literal> and <literal>OUTER</literal> join types, a
        join condition must be specified, namely exactly one of
        <literal>NATURAL</literal>, <literal>ON <replaceable
        class="parameter">join_condition</replaceable></literal>, or
        <literal>USING (<replaceable
        class="parameter">join_column</replaceable> [, ...])</literal>.
        See below for the meaning.  For <literal>CROSS JOIN</literal>,
        none of these clauses can appear.
       </para>

       <para>
        A <literal>JOIN</literal> clause combines two <literal>FROM</literal>
        items, which for convenience we will refer to as <quote>tables</quote>,
        though in reality they can be any type of <literal>FROM</literal> item.
        Use parentheses if necessary to determine the order of nesting.
        In the absence of parentheses, <literal>JOIN</literal>s nest
        left-to-right.  In any case <literal>JOIN</literal> binds more
        tightly than the commas separating <literal>FROM</literal>-list items.
       </para>

       <para><literal>CROSS JOIN</literal> and <literal>INNER JOIN</literal>
        produce a simple Cartesian product, the same result as you get from
        listing the two tables at the top level of <literal>FROM</literal>,
        but restricted by the join condition (if any).
        <literal>CROSS JOIN</literal> is equivalent to <literal>INNER JOIN ON
        (TRUE)</literal>, that is, no rows are removed by qualification.
        These join types are just a notational convenience, since they
        do nothing you couldn't do with plain <literal>FROM</literal> and
        <literal>WHERE</literal>.
       </para>

       <para><literal>LEFT OUTER JOIN</literal> returns all rows in the qualified
        Cartesian product (i.e., all combined rows that pass its join
        condition), plus one copy of each row in the left-hand table
        for which there was no right-hand row that passed the join
        condition.  This left-hand row is extended to the full width
        of the joined table by inserting null values for the
        right-hand columns.  Note that only the <literal>JOIN</literal>
        clause's own condition is considered while deciding which rows
        have matches.  Outer conditions are applied afterwards.
       </para>

       <para>
        Conversely, <literal>RIGHT OUTER JOIN</literal> returns all the
        joined rows, plus one row for each unmatched right-hand row
        (extended with nulls on the left).  This is just a notational
        convenience, since you could convert it to a <literal>LEFT
        OUTER JOIN</literal> by switching the left and right tables.
       </para>

       <para><literal>FULL OUTER JOIN</literal> returns all the joined rows, plus
        one row for each unmatched left-hand row (extended with nulls
        on the right), plus one row for each unmatched right-hand row
        (extended with nulls on the left).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>ON <replaceable class="parameter">join_condition</replaceable></literal></term>
      <listitem>
       <para><replaceable class="parameter">join_condition</replaceable> is
        an expression resulting in a value of type
        <type>boolean</type> (similar to a <literal>WHERE</literal>
        clause) that specifies which rows in a join are considered to
        match.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>USING ( <replaceable class="parameter">join_column</replaceable> [, ...] ) [ AS <replaceable class="parameter">join_using_alias</replaceable> ]</literal></term>
      <listitem>
       <para>
        A clause of the form <literal>USING ( a, b, ... )</literal> is
        shorthand for <literal>ON left_table.a = right_table.a AND
        left_table.b = right_table.b ...</literal>.  Also,
        <literal>USING</literal> implies that only one of each pair of
        equivalent columns will be included in the join output, not
        both.
       </para>

       <para>
        If a <replaceable class="parameter">join_using_alias</replaceable>
        name is specified, it provides a table alias for the join columns.
        Only the join columns listed in the <literal>USING</literal> clause
        are addressable by this name.  Unlike a regular <replaceable
        class="parameter">alias</replaceable>, this does not hide the names of
        the joined tables from the rest of the query.  Also unlike a regular
        <replaceable class="parameter">alias</replaceable>, you cannot write a
        column alias list &mdash; the output names of the join columns are the
        same as they appear in the <literal>USING</literal> list.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NATURAL</literal></term>
      <listitem>
       <para>
        <literal>NATURAL</literal> is shorthand for a
        <literal>USING</literal> list that mentions all columns in the two
        tables that have matching names.  If there are no common
        column names, <literal>NATURAL</literal> is equivalent
        to <literal>ON TRUE</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>LATERAL</literal></term>
      <listitem>
       <para>
        The <literal>LATERAL</literal> key word can precede a
        sub-<command>SELECT</command> <literal>FROM</literal> item.  This allows the
        sub-<command>SELECT</command> to refer to columns of <literal>FROM</literal>
        items that appear before it in the <literal>FROM</literal> list.  (Without
        <literal>LATERAL</literal>, each sub-<command>SELECT</command> is
        evaluated independently and so cannot cross-reference any other
        <literal>FROM</literal> item.)
       </para>

       <para><literal>LATERAL</literal> can also precede a function-call
        <literal>FROM</literal> item, but in this case it is a noise word, because
        the function expression can refer to earlier <literal>FROM</literal> items
        in any case.
       </para>

       <para>
        A <literal>LATERAL</literal> item can appear at top level in the
        <literal>FROM</literal> list, or within a <literal>JOIN</literal> tree.  In the
        latter case it can also refer to any items that are on the left-hand
        side of a <literal>JOIN</literal> that it is on the right-hand side of.
       </para>

       <para>
        When a <literal>FROM</literal> item contains <literal>LATERAL</literal>
        cross-references, evaluation proceeds as follows: for each row of the
        <literal>FROM</literal> item providing the cross-referenced column(s), or
        set of rows of multiple <literal>FROM</literal> items providing the
        columns, the <literal>LATERAL</literal> item is evaluated using that
        row or row set's values of the columns.  The resulting row(s) are
        joined as usual with the rows they were computed from.  This is
        repeated for each row or set of rows from the column source table(s).
       </para>

       <para>
        The column source table(s) must be <literal>INNER</literal> or
        <literal>LEFT</literal> joined to the <literal>LATERAL</literal> item, else
        there would not be a well-defined set of rows from which to compute
        each set of rows for the <literal>LATERAL</literal> item.  Thus,
        although a construct such as <literal><replaceable>X</replaceable> RIGHT JOIN
        LATERAL <replaceable>Y</replaceable></literal> is syntactically valid, it is
        not actually allowed for <replaceable>Y</replaceable> to reference
        <replaceable>X</replaceable>.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="sql-where" xreflabel="WHERE Clause">
   <title><literal>WHERE</literal> Clause</title>

   <para>
    The optional <literal>WHERE</literal> clause has the general form
<synopsis>
WHERE <replaceable class="parameter">condition</replaceable>
</synopsis>
    where <replaceable class="parameter">condition</replaceable> is
    any expression that evaluates to a result of type
    <type>boolean</type>.  Any row that does not satisfy this
    condition will be eliminated from the output.  A row satisfies the
    condition if it returns true when the actual row values are
    substituted for any variable references.
   </para>
  </refsect2>

  <refsect2 id="sql-groupby" xreflabel="GROUP BY Clause">
   <title><literal>GROUP BY</literal> Clause</title>

   <para>
    The optional <literal>GROUP BY</literal> clause has the general form
<synopsis>
GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...]
</synopsis>
   </para>

   <para>
    <literal>GROUP BY</literal> will condense into a single row all
    selected rows that share the same values for the grouped
    expressions.  An <replaceable
    class="parameter">expression</replaceable> used inside a
    <replaceable class="parameter">grouping_element</replaceable>
    can be an input column name, or the name or ordinal number of an
    output column (<command>SELECT</command> list item), or an arbitrary
    expression formed from input-column values.  In case of ambiguity,
    a <literal>GROUP BY</literal> name will be interpreted as an
    input-column name rather than an output column name.
   </para>

   <para>
    If any of <literal>GROUPING SETS</literal>, <literal>ROLLUP</literal> or
    <literal>CUBE</literal> are present as grouping elements, then the
    <literal>GROUP BY</literal> clause as a whole defines some number of
    independent <replaceable>grouping sets</replaceable>.  The effect of this is
    equivalent to constructing a <literal>UNION ALL</literal> between
    subqueries with the individual grouping sets as their
    <literal>GROUP BY</literal> clauses.  The optional <literal>DISTINCT</literal>
    clause removes duplicate sets before processing; it does <emphasis>not</emphasis>
    transform the <literal>UNION ALL</literal> into a <literal>UNION DISTINCT</literal>.
    For further details on the handling
    of grouping sets see <xref linkend="queries-grouping-sets"/>.
   </para>

   <para>
    Aggregate functions, if any are used, are computed across all rows
    making up each group, producing a separate value for each group.
    (If there are aggregate functions but no <literal>GROUP BY</literal>
    clause, the query is treated as having a single group comprising all
    the selected rows.)
    The set of rows fed to each aggregate function can be further filtered by
    attaching a <literal>FILTER</literal> clause to the aggregate function
    call; see <xref linkend="syntax-aggregates"/> for more information.  When
    a <literal>FILTER</literal> clause is present, only those rows matching it
    are included in the input to that aggregate function.
   </para>

   <para>
    When <literal>GROUP BY</literal> is present,
    or any aggregate functions are present, it is not valid for
    the <command>SELECT</command> list expressions to refer to
    ungrouped columns except within aggregate functions or when the
    ungrouped column is functionally dependent on the grouped columns,
    since there would otherwise be more than one possible value to
    return for an ungrouped column.  A functional dependency exists if
    the grouped columns (or a subset thereof) are the primary key of
    the table containing the ungrouped column.
   </para>

   <para>
    Keep in mind that all aggregate functions are evaluated before
    evaluating any <quote>scalar</quote> expressions in the <literal>HAVING</literal>
    clause or <literal>SELECT</literal> list.  This means that, for example,
    a <literal>CASE</literal> expression cannot be used to skip evaluation of
    an aggregate function; see <xref linkend="syntax-express-eval"/>.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal> and <literal>FOR KEY SHARE</literal> cannot be
    specified with <literal>GROUP BY</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-having" xreflabel="HAVING Clause">
   <title><literal>HAVING</literal> Clause</title>

   <para>
    The optional <literal>HAVING</literal> clause has the general form
<synopsis>
HAVING <replaceable class="parameter">condition</replaceable>
</synopsis>
    where <replaceable class="parameter">condition</replaceable> is
    the same as specified for the <literal>WHERE</literal> clause.
   </para>

   <para>
    <literal>HAVING</literal> eliminates group rows that do not
    satisfy the condition.  <literal>HAVING</literal> is different
    from <literal>WHERE</literal>: <literal>WHERE</literal> filters
    individual rows before the application of <literal>GROUP
    BY</literal>, while <literal>HAVING</literal> filters group rows
    created by <literal>GROUP BY</literal>.  Each column referenced in
    <replaceable class="parameter">condition</replaceable> must
    unambiguously reference a grouping column, unless the reference
    appears within an aggregate function or the ungrouped column is
    functionally dependent on the grouping columns.
   </para>

   <para>
    The presence of <literal>HAVING</literal> turns a query into a grouped
    query even if there is no <literal>GROUP BY</literal> clause.  This is the
    same as what happens when the query contains aggregate functions but
    no <literal>GROUP BY</literal> clause.  All the selected rows are considered to
    form a single group, and the <command>SELECT</command> list and
    <literal>HAVING</literal> clause can only reference table columns from
    within aggregate functions.  Such a query will emit a single row if the
    <literal>HAVING</literal> condition is true, zero rows if it is not true.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal> and <literal>FOR KEY SHARE</literal> cannot be
    specified with <literal>HAVING</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-window" xreflabel="WINDOW Clause">
   <title><literal>WINDOW</literal> Clause</title>

   <para>
    The optional <literal>WINDOW</literal> clause has the general form
<synopsis>
WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...]
</synopsis>
    where <replaceable class="parameter">window_name</replaceable> is
    a name that can be referenced from <literal>OVER</literal> clauses or
    subsequent window definitions, and
    <replaceable class="parameter">window_definition</replaceable> is
<synopsis>
[ <replaceable class="parameter">existing_window_name</replaceable> ]
[ PARTITION BY <replaceable class="parameter">expression</replaceable> [, ...] ]
[ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...] ]
[ <replaceable class="parameter">frame_clause</replaceable> ]
</synopsis>
   </para>

   <para>
    If an <replaceable class="parameter">existing_window_name</replaceable>
    is specified it must refer to an earlier entry in the <literal>WINDOW</literal>
    list; the new window copies its partitioning clause from that entry,
    as well as its ordering clause if any.  In this case the new window cannot
    specify its own <literal>PARTITION BY</literal> clause, and it can specify
    <literal>ORDER BY</literal> only if the copied window does not have one.
    The new window always uses its own frame clause; the copied window
    must not specify a frame clause.
   </para>

   <para>
    The elements of the <literal>PARTITION BY</literal> list are interpreted in
    much the same fashion as elements of a <link
    linkend="sql-groupby"><literal>GROUP BY</literal></link> clause, except that
    they are always simple expressions and never the name or number of an
    output column.
    Another difference is that these expressions can contain aggregate
    function calls, which are not allowed in a regular <literal>GROUP BY</literal>
    clause.  They are allowed here because windowing occurs after grouping
    and aggregation.
   </para>

   <para>
    Similarly, the elements of the <literal>ORDER BY</literal> list are interpreted
    in much the same fashion as elements of a statement-level <link
    linkend="sql-orderby"><literal>ORDER BY</literal></link> clause, except that
    the expressions are always taken as simple expressions and never the name
    or number of an output column.
   </para>

   <para>
    The optional <replaceable class="parameter">frame_clause</replaceable> defines
    the <firstterm>window frame</firstterm> for window functions that depend on the
    frame (not all do).  The window frame is a set of related rows for
    each row of the query (called the <firstterm>current row</firstterm>).
    The <replaceable class="parameter">frame_clause</replaceable> can be one of

<synopsis>
{ RANGE | ROWS | GROUPS } <replaceable>frame_start</replaceable> [ <replaceable>frame_exclusion</replaceable> ]
{ RANGE | ROWS | GROUPS } BETWEEN <replaceable>frame_start</replaceable> AND <replaceable>frame_end</replaceable> [ <replaceable>frame_exclusion</replaceable> ]
</synopsis>

    where <replaceable>frame_start</replaceable>
    and <replaceable>frame_end</replaceable> can be one of

<synopsis>
UNBOUNDED PRECEDING
<replaceable>offset</replaceable> PRECEDING
CURRENT ROW
<replaceable>offset</replaceable> FOLLOWING
UNBOUNDED FOLLOWING
</synopsis>

    and <replaceable>frame_exclusion</replaceable> can be one of

<synopsis>
EXCLUDE CURRENT ROW
EXCLUDE GROUP
EXCLUDE TIES
EXCLUDE NO OTHERS
</synopsis>

    If <replaceable>frame_end</replaceable> is omitted it defaults to <literal>CURRENT
    ROW</literal>.  Restrictions are that
    <replaceable>frame_start</replaceable> cannot be <literal>UNBOUNDED FOLLOWING</literal>,
    <replaceable>frame_end</replaceable> cannot be <literal>UNBOUNDED PRECEDING</literal>,
    and the <replaceable>frame_end</replaceable> choice cannot appear earlier in the
    above list of <replaceable>frame_start</replaceable>
    and <replaceable>frame_end</replaceable> options than
    the <replaceable>frame_start</replaceable> choice does &mdash; for example
    <literal>RANGE BETWEEN CURRENT ROW AND <replaceable>offset</replaceable>
    PRECEDING</literal> is not allowed.
   </para>

   <para>
    The default framing option is <literal>RANGE UNBOUNDED PRECEDING</literal>,
    which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND
    CURRENT ROW</literal>; it sets the frame to be all rows from the partition start
    up through the current row's last <firstterm>peer</firstterm> (a row
    that the window's <literal>ORDER BY</literal> clause considers
    equivalent to the current row; all rows are peers if there
    is no <literal>ORDER BY</literal>).
    In general, <literal>UNBOUNDED PRECEDING</literal> means that the frame
    starts with the first row of the partition, and similarly
    <literal>UNBOUNDED FOLLOWING</literal> means that the frame ends with the last
    row of the partition, regardless
    of <literal>RANGE</literal>, <literal>ROWS</literal>
    or <literal>GROUPS</literal> mode.
    In <literal>ROWS</literal> mode, <literal>CURRENT ROW</literal> means
    that the frame starts or ends with the current row; but
    in <literal>RANGE</literal> or <literal>GROUPS</literal> mode it means
    that the frame starts or ends with the current row's first or last peer
    in the <literal>ORDER BY</literal> ordering.
    The <replaceable>offset</replaceable> <literal>PRECEDING</literal> and
    <replaceable>offset</replaceable> <literal>FOLLOWING</literal> options
    vary in meaning depending on the frame mode.
    In <literal>ROWS</literal> mode, the <replaceable>offset</replaceable>
    is an integer indicating that the frame starts or ends that many rows
    before or after the current row.
    In <literal>GROUPS</literal> mode, the <replaceable>offset</replaceable>
    is an integer indicating that the frame starts or ends that many peer
    groups before or after the current row's peer group, where
    a <firstterm>peer group</firstterm> is a group of rows that are
    equivalent according to the window's <literal>ORDER BY</literal> clause.
    In <literal>RANGE</literal> mode, use of
    an <replaceable>offset</replaceable> option requires that there be
    exactly one <literal>ORDER BY</literal> column in the window definition.
    Then the frame contains those rows whose ordering column value is no
    more than <replaceable>offset</replaceable> less than
    (for <literal>PRECEDING</literal>) or more than
    (for <literal>FOLLOWING</literal>) the current row's ordering column
    value.  In these cases the data type of
    the <replaceable>offset</replaceable> expression depends on the data
    type of the ordering column.  For numeric ordering columns it is
    typically of the same type as the ordering column, but for datetime
    ordering columns it is an <type>interval</type>.
    In all these cases, the value of the <replaceable>offset</replaceable>
    must be non-null and non-negative.  Also, while
    the <replaceable>offset</replaceable> does not have to be a simple
    constant, it cannot contain variables, aggregate functions, or window
    functions.
   </para>

   <para>
    The <replaceable>frame_exclusion</replaceable> option allows rows around
    the current row to be excluded from the frame, even if they would be
    included according to the frame start and frame end options.
    <literal>EXCLUDE CURRENT ROW</literal> excludes the current row from the
    frame.
    <literal>EXCLUDE GROUP</literal> excludes the current row and its
    ordering peers from the frame.
    <literal>EXCLUDE TIES</literal> excludes any peers of the current
    row from the frame, but not the current row itself.
    <literal>EXCLUDE NO OTHERS</literal> simply specifies explicitly the
    default behavior of not excluding the current row or its peers.
   </para>

   <para>
    Beware that the <literal>ROWS</literal> mode can produce unpredictable
    results if the <literal>ORDER BY</literal> ordering does not order the rows
    uniquely.  The <literal>RANGE</literal> and <literal>GROUPS</literal>
    modes are designed to ensure that rows that are peers in
    the <literal>ORDER BY</literal> ordering are treated alike: all rows of
    a given peer group will be in the frame or excluded from it.
   </para>

   <para>
    The purpose of a <literal>WINDOW</literal> clause is to specify the
    behavior of <firstterm>window functions</firstterm> appearing in the query's
    <link linkend="sql-select-list"><command>SELECT</command> list</link> or
    <link linkend="sql-orderby"><literal>ORDER BY</literal></link> clause.
    These functions
    can reference the <literal>WINDOW</literal> clause entries by name
    in their <literal>OVER</literal> clauses.  A <literal>WINDOW</literal> clause
    entry does not have to be referenced anywhere, however; if it is not
    used in the query it is simply ignored.  It is possible to use window
    functions without any <literal>WINDOW</literal> clause at all, since
    a window function call can specify its window definition directly in
    its <literal>OVER</literal> clause.  However, the <literal>WINDOW</literal>
    clause saves typing when the same window definition is needed for more
    than one window function.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal> and <literal>FOR KEY SHARE</literal> cannot be
    specified with <literal>WINDOW</literal>.
   </para>

   <para>
    Window functions are described in detail in
    <xref linkend="tutorial-window"/>,
    <xref linkend="syntax-window-functions"/>, and
    <xref linkend="queries-window"/>.
   </para>
  </refsect2>

  <refsect2 id="sql-select-list" xreflabel="SELECT List">
   <title><command>SELECT</command> List</title>

   <para>
    The <command>SELECT</command> list (between the key words
    <literal>SELECT</literal> and <literal>FROM</literal>) specifies expressions
    that form the output rows of the <command>SELECT</command>
    statement.  The expressions can (and usually do) refer to columns
    computed in the <literal>FROM</literal> clause.
   </para>

   <para>
    Just as in a table, every output column of a <command>SELECT</command>
    has a name.  In a simple <command>SELECT</command> this name is just
    used to label the column for display, but when the <command>SELECT</command>
    is a sub-query of a larger query, the name is seen by the larger query
    as the column name of the virtual table produced by the sub-query.
    To specify the name to use for an output column, write
    <literal>AS</literal> <replaceable class="parameter">output_name</replaceable>
    after the column's expression.  (You can omit <literal>AS</literal>,
    but only if the desired output name does not match any
    <productname>PostgreSQL</productname> keyword (see <xref
    linkend="sql-keywords-appendix"/>).  For protection against possible
    future keyword additions, it is recommended that you always either
    write <literal>AS</literal> or double-quote the output name.)
    If you do not specify a column name, a name is chosen automatically
    by <productname>PostgreSQL</productname>.  If the column's expression
    is a simple column reference then the chosen name is the same as that
    column's name.  In more complex cases a function or type name may be
    used, or the system may fall back on a generated name such as
    <literal>?column?</literal>.
   </para>

   <para>
    An output column's name can be used to refer to the column's value in
    <literal>ORDER BY</literal> and <literal>GROUP BY</literal> clauses, but not in the
    <literal>WHERE</literal> or <literal>HAVING</literal> clauses; there you must write
    out the expression instead.
   </para>

   <para>
    Instead of an expression, <literal>*</literal> can be written in
    the output list as a shorthand for all the columns of the selected
    rows.  Also, you can write <literal><replaceable
    class="parameter">table_name</replaceable>.*</literal> as a
    shorthand for the columns coming from just that table.  In these
    cases it is not possible to specify new names with <literal>AS</literal>;
    the output column names will be the same as the table columns' names.
   </para>

   <para>
    According to the SQL standard, the expressions in the output list should
    be computed before applying <literal>DISTINCT</literal>, <literal>ORDER
    BY</literal>, or <literal>LIMIT</literal>.  This is obviously necessary
    when using <literal>DISTINCT</literal>, since otherwise it's not clear
    what values are being made distinct.  However, in many cases it is
    convenient if output expressions are computed after <literal>ORDER
    BY</literal> and <literal>LIMIT</literal>; particularly if the output list
    contains any volatile or expensive functions.  With that behavior, the
    order of function evaluations is more intuitive and there will not be
    evaluations corresponding to rows that never appear in the output.
    <productname>PostgreSQL</productname> will effectively evaluate output expressions
    after sorting and limiting, so long as those expressions are not
    referenced in <literal>DISTINCT</literal>, <literal>ORDER BY</literal>
    or <literal>GROUP BY</literal>.  (As a counterexample, <literal>SELECT
    f(x) FROM tab ORDER BY 1</literal> clearly must evaluate <function>f(x)</function>
    before sorting.)  Output expressions that contain set-returning functions
    are effectively evaluated after sorting and before limiting, so
    that <literal>LIMIT</literal> will act to cut off the output from a
    set-returning function.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> versions before 9.6 did not provide any
     guarantees about the timing of evaluation of output expressions versus
     sorting and limiting; it depended on the form of the chosen query plan.
    </para>
   </note>
  </refsect2>

  <refsect2 id="sql-distinct" xreflabel="DISTINCT Clause">
   <title><literal>DISTINCT</literal> Clause</title>

   <para>
    If <literal>SELECT DISTINCT</literal> is specified, all duplicate rows are
    removed from the result set (one row is kept from each group of
    duplicates).  <literal>SELECT ALL</literal> specifies the opposite: all rows are
    kept; that is the default.
   </para>

   <para>
    <literal>SELECT DISTINCT ON ( <replaceable
    class="parameter">expression</replaceable> [, ...] )</literal>
    keeps only the first row of each set of rows where the given
    expressions evaluate to equal.  The <literal>DISTINCT ON</literal>
    expressions are interpreted using the same rules as for
    <literal>ORDER BY</literal> (see above).  Note that the <quote>first
    row</quote> of each set is unpredictable unless <literal>ORDER
    BY</literal> is used to ensure that the desired row appears first.  For
    example:
<programlisting>
SELECT DISTINCT ON (location) location, time, report
    FROM weather_reports
    ORDER BY location, time DESC;
</programlisting>
    retrieves the most recent weather report for each location.  But
    if we had not used <literal>ORDER BY</literal> to force descending order
    of time values for each location, we'd have gotten a report from
    an unpredictable time for each location.
   </para>

   <para>
    The <literal>DISTINCT ON</literal> expression(s) must match the leftmost
    <literal>ORDER BY</literal> expression(s).  The <literal>ORDER BY</literal> clause
    will normally contain additional expression(s) that determine the
    desired precedence of rows within each <literal>DISTINCT ON</literal> group.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal> and <literal>FOR KEY SHARE</literal> cannot be
    specified with <literal>DISTINCT</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-union" xreflabel="UNION Clause">
   <title><literal>UNION</literal> Clause</title>

   <para>
    The <literal>UNION</literal> clause has this general form:
<synopsis>
<replaceable class="parameter">select_statement</replaceable> UNION [ ALL | DISTINCT ] <replaceable class="parameter">select_statement</replaceable>
</synopsis><replaceable class="parameter">select_statement</replaceable> is
    any <command>SELECT</command> statement without an <literal>ORDER
    BY</literal>, <literal>LIMIT</literal>, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal>, or <literal>FOR KEY SHARE</literal> clause.
    (<literal>ORDER BY</literal> and <literal>LIMIT</literal> can be attached to a
    subexpression if it is enclosed in parentheses.  Without
    parentheses, these clauses will be taken to apply to the result of
    the <literal>UNION</literal>, not to its right-hand input
    expression.)
   </para>

   <para>
    The <literal>UNION</literal> operator computes the set union of
    the rows returned by the involved <command>SELECT</command>
    statements.  A row is in the set union of two result sets if it
    appears in at least one of the result sets.  The two
    <command>SELECT</command> statements that represent the direct
    operands of the <literal>UNION</literal> must produce the same
    number of columns, and corresponding columns must be of compatible
    data types.
   </para>

   <para>
    The result of <literal>UNION</literal> does not contain any duplicate
    rows unless the <literal>ALL</literal> option is specified.
    <literal>ALL</literal> prevents elimination of duplicates.  (Therefore,
    <literal>UNION ALL</literal> is usually significantly quicker than
    <literal>UNION</literal>; use <literal>ALL</literal> when you can.)
    <literal>DISTINCT</literal> can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   </para>

   <para>
    Multiple <literal>UNION</literal> operators in the same
    <command>SELECT</command> statement are evaluated left to right,
    unless otherwise indicated by parentheses.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>, <literal>FOR SHARE</literal> and
    <literal>FOR KEY SHARE</literal> cannot be
    specified either for a <literal>UNION</literal> result or for any input of a
    <literal>UNION</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-intersect" xreflabel="INTERSECT Clause">
   <title><literal>INTERSECT</literal> Clause</title>

   <para>
    The <literal>INTERSECT</literal> clause has this general form:
<synopsis>
<replaceable class="parameter">select_statement</replaceable> INTERSECT [ ALL | DISTINCT ] <replaceable class="parameter">select_statement</replaceable>
</synopsis><replaceable class="parameter">select_statement</replaceable> is
    any <command>SELECT</command> statement without an <literal>ORDER
    BY</literal>, <literal>LIMIT</literal>, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal>, or <literal>FOR KEY SHARE</literal> clause.
   </para>

   <para>
    The <literal>INTERSECT</literal> operator computes the set
    intersection of the rows returned by the involved
    <command>SELECT</command> statements.  A row is in the
    intersection of two result sets if it appears in both result sets.
   </para>

   <para>
    The result of <literal>INTERSECT</literal> does not contain any
    duplicate rows unless the <literal>ALL</literal> option is specified.
    With <literal>ALL</literal>, a row that has <replaceable>m</replaceable> duplicates in the
    left table and <replaceable>n</replaceable> duplicates in the right table will appear
    min(<replaceable>m</replaceable>,<replaceable>n</replaceable>) times in the result set.
    <literal>DISTINCT</literal> can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   </para>

   <para>
    Multiple <literal>INTERSECT</literal> operators in the same
    <command>SELECT</command> statement are evaluated left to right,
    unless parentheses dictate otherwise.
    <literal>INTERSECT</literal> binds more tightly than
    <literal>UNION</literal>.  That is, <literal>A UNION B INTERSECT
    C</literal> will be read as <literal>A UNION (B INTERSECT
    C)</literal>.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>, <literal>FOR SHARE</literal> and
    <literal>FOR KEY SHARE</literal> cannot be
    specified either for an <literal>INTERSECT</literal> result or for any input of
    an <literal>INTERSECT</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-except" xreflabel="EXCEPT Clause">
   <title><literal>EXCEPT</literal> Clause</title>

   <para>
    The <literal>EXCEPT</literal> clause has this general form:
<synopsis>
<replaceable class="parameter">select_statement</replaceable> EXCEPT [ ALL | DISTINCT ] <replaceable class="parameter">select_statement</replaceable>
</synopsis><replaceable class="parameter">select_statement</replaceable> is
    any <command>SELECT</command> statement without an <literal>ORDER
    BY</literal>, <literal>LIMIT</literal>, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>,
    <literal>FOR SHARE</literal>, or <literal>FOR KEY SHARE</literal> clause.
   </para>

   <para>
    The <literal>EXCEPT</literal> operator computes the set of rows
    that are in the result of the left <command>SELECT</command>
    statement but not in the result of the right one.
   </para>

   <para>
    The result of <literal>EXCEPT</literal> does not contain any
    duplicate rows unless the <literal>ALL</literal> option is specified.
    With <literal>ALL</literal>, a row that has <replaceable>m</replaceable> duplicates in the
    left table and <replaceable>n</replaceable> duplicates in the right table will appear
    max(<replaceable>m</replaceable>-<replaceable>n</replaceable>,0) times in the result set.
    <literal>DISTINCT</literal> can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   </para>

   <para>
    Multiple <literal>EXCEPT</literal> operators in the same
    <command>SELECT</command> statement are evaluated left to right,
    unless parentheses dictate otherwise.  <literal>EXCEPT</literal> binds at
    the same level as <literal>UNION</literal>.
   </para>

   <para>
    Currently, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>, <literal>FOR SHARE</literal> and
    <literal>FOR KEY SHARE</literal> cannot be
    specified either for an <literal>EXCEPT</literal> result or for any input of
    an <literal>EXCEPT</literal>.
   </para>
  </refsect2>

  <refsect2 id="sql-orderby" xreflabel="ORDER BY Clause">
   <title><literal>ORDER BY</literal> Clause</title>

   <para>
    The optional <literal>ORDER BY</literal> clause has this general form:
<synopsis>
ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...]
</synopsis>
    The <literal>ORDER BY</literal> clause causes the result rows to
    be sorted according to the specified expression(s).  If two rows are
    equal according to the leftmost expression, they are compared
    according to the next expression and so on.  If they are equal
    according to all specified expressions, they are returned in
    an implementation-dependent order.
   </para>

   <para>
    Each <replaceable class="parameter">expression</replaceable> can be the
    name or ordinal number of an output column
    (<command>SELECT</command> list item), or it can be an arbitrary
    expression formed from input-column values.
   </para>

   <para>
    The ordinal number refers to the ordinal (left-to-right) position
    of the output column. This feature makes it possible to define an
    ordering on the basis of a column that does not have a unique
    name.  This is never absolutely necessary because it is always
    possible to assign a name to an output column using the
    <literal>AS</literal> clause.
   </para>

   <para>
    It is also possible to use arbitrary expressions in the
    <literal>ORDER BY</literal> clause, including columns that do not
    appear in the <command>SELECT</command> output list.  Thus the
    following statement is valid:
<programlisting>
SELECT name FROM distributors ORDER BY code;
</programlisting>
    A limitation of this feature is that an <literal>ORDER BY</literal>
    clause applying to the result of a <literal>UNION</literal>,
    <literal>INTERSECT</literal>, or <literal>EXCEPT</literal> clause can only
    specify an output column name or number, not an expression.
   </para>

   <para>
    If an <literal>ORDER BY</literal> expression is a simple name that
    matches both an output column name and an input column name,
    <literal>ORDER BY</literal> will interpret it as the output column name.
    This is the opposite of the choice that <literal>GROUP BY</literal> will
    make in the same situation.  This inconsistency is made to be
    compatible with the SQL standard.
   </para>

   <para>
    Optionally one can add the key word <literal>ASC</literal> (ascending) or
    <literal>DESC</literal> (descending) after any expression in the
    <literal>ORDER BY</literal> clause.  If not specified, <literal>ASC</literal> is
    assumed by default.  Alternatively, a specific ordering operator
    name can be specified in the <literal>USING</literal> clause.
    An ordering operator must be a less-than or greater-than
    member of some B-tree operator family.
    <literal>ASC</literal> is usually equivalent to <literal>USING &lt;</literal> and
    <literal>DESC</literal> is usually equivalent to <literal>USING &gt;</literal>.
    (But the creator of a user-defined data type can define exactly what the
    default sort ordering is, and it might correspond to operators with other
    names.)
   </para>

   <para>
    If <literal>NULLS LAST</literal> is specified, null values sort after all
    non-null values; if <literal>NULLS FIRST</literal> is specified, null values
    sort before all non-null values.  If neither is specified, the default
    behavior is <literal>NULLS LAST</literal> when <literal>ASC</literal> is specified
    or implied, and <literal>NULLS FIRST</literal> when <literal>DESC</literal> is specified
    (thus, the default is to act as though nulls are larger than non-nulls).
    When <literal>USING</literal> is specified, the default nulls ordering depends
    on whether the operator is a less-than or greater-than operator.
   </para>

   <para>
    Note that ordering options apply only to the expression they follow;
    for example <literal>ORDER BY x, y DESC</literal> does not mean
    the same thing as <literal>ORDER BY x DESC, y DESC</literal>.
   </para>

   <para>
    Character-string data is sorted according to the collation that applies
    to the column being sorted.  That can be overridden at need by including
    a <literal>COLLATE</literal> clause in the
    <replaceable class="parameter">expression</replaceable>, for example
    <literal>ORDER BY mycolumn COLLATE "en_US"</literal>.
    For more information see <xref linkend="sql-syntax-collate-exprs"/> and
    <xref linkend="collation"/>.
   </para>
  </refsect2>

  <refsect2 id="sql-limit" xreflabel="LIMIT Clause">
   <title><literal>LIMIT</literal> Clause</title>

   <para>
    The <literal>LIMIT</literal> clause consists of two independent
    sub-clauses:
<synopsis>
LIMIT { <replaceable class="parameter">count</replaceable> | ALL }
OFFSET <replaceable class="parameter">start</replaceable>
</synopsis>
    The parameter <replaceable class="parameter">count</replaceable> specifies the
    maximum number of rows to return, while <replaceable
    class="parameter">start</replaceable> specifies the number of rows
    to skip before starting to return rows.  When both are specified,
    <replaceable class="parameter">start</replaceable> rows are skipped
    before starting to count the <replaceable
    class="parameter">count</replaceable> rows to be returned.
   </para>

   <para>
    If the <replaceable class="parameter">count</replaceable> expression
    evaluates to NULL, it is treated as <literal>LIMIT ALL</literal>, i.e., no
    limit.  If <replaceable class="parameter">start</replaceable> evaluates
    to NULL, it is treated the same as <literal>OFFSET 0</literal>.
   </para>

   <para>
    SQL:2008 introduced a different syntax to achieve the same result,
    which <productname>PostgreSQL</productname> also supports.  It is:
<synopsis>
OFFSET <replaceable class="parameter">start</replaceable> { ROW | ROWS }
FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } { ONLY | WITH TIES }
</synopsis>
    In this syntax, the <replaceable class="parameter">start</replaceable>
    or <replaceable class="parameter">count</replaceable> value is required by
    the standard to be a literal constant, a parameter, or a variable name;
    as a <productname>PostgreSQL</productname> extension, other expressions
    are allowed, but will generally need to be enclosed in parentheses to avoid
    ambiguity.
    If <replaceable class="parameter">count</replaceable> is
    omitted in a <literal>FETCH</literal> clause, it defaults to 1.
    The <literal>WITH TIES</literal> option is used to return any additional
    rows that tie for the last place in the result set according to
    the <literal>ORDER BY</literal> clause; <literal>ORDER BY</literal>
    is mandatory in this case, and <literal>SKIP LOCKED</literal> is
    not allowed.
    <literal>ROW</literal> and <literal>ROWS</literal> as well as
    <literal>FIRST</literal> and <literal>NEXT</literal> are noise
    words that don't influence the effects of these clauses.
    According to the standard, the <literal>OFFSET</literal> clause must come
    before the <literal>FETCH</literal> clause if both are present; but
    <productname>PostgreSQL</productname> is laxer and allows either order.
   </para>

   <para>
    When using <literal>LIMIT</literal>, it is a good idea to use an
    <literal>ORDER BY</literal> clause that constrains the result rows into a
    unique order.  Otherwise you will get an unpredictable subset of
    the query's rows &mdash; you might be asking for the tenth through
    twentieth rows, but tenth through twentieth in what ordering?  You
    don't know what ordering unless you specify <literal>ORDER BY</literal>.
   </para>

   <para>
    The query planner takes <literal>LIMIT</literal> into account when
    generating a query plan, so you are very likely to get different
    plans (yielding different row orders) depending on what you use
    for <literal>LIMIT</literal> and <literal>OFFSET</literal>.  Thus, using
    different <literal>LIMIT</literal>/<literal>OFFSET</literal> values to select
    different subsets of a query result <emphasis>will give
    inconsistent results</emphasis> unless you enforce a predictable
    result ordering with <literal>ORDER BY</literal>.  This is not a bug; it
    is an inherent consequence of the fact that SQL does not promise
    to deliver the results of a query in any particular order unless
    <literal>ORDER BY</literal> is used to constrain the order.
   </para>

   <para>
    It is even possible for repeated executions of the same <literal>LIMIT</literal>
    query to return different subsets of the rows of a table, if there
    is not an <literal>ORDER BY</literal> to enforce selection of a deterministic
    subset.  Again, this is not a bug; determinism of the results is
    simply not guaranteed in such a case.
   </para>
  </refsect2>

  <refsect2 id="sql-for-update-share" xreflabel="The Locking Clause">
   <title>The Locking Clause</title>

   <para>
    <literal>FOR UPDATE</literal>, <literal>FOR NO KEY UPDATE</literal>, <literal>FOR SHARE</literal>
    and <literal>FOR KEY SHARE</literal>
    are <firstterm>locking clauses</firstterm>; they affect how <literal>SELECT</literal>
    locks rows as they are obtained from the table.
   </para>

   <para>
    The locking clause has the general form

<synopsis>
FOR <replaceable>lock_strength</replaceable> [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ]
</synopsis>

    where <replaceable>lock_strength</replaceable> can be one of

<synopsis>
UPDATE
NO KEY UPDATE
SHARE
KEY SHARE
</synopsis>
   </para>

   <para>
    For more information on each row-level lock mode, refer to
    <xref linkend="locking-rows"/>.
   </para>

   <para>
    To prevent the operation from waiting for other transactions to commit,
    use either the <literal>NOWAIT</literal> or <literal>SKIP LOCKED</literal>
    option.  With <literal>NOWAIT</literal>, the statement reports an error, rather
    than waiting, if a selected row cannot be locked immediately.
    With <literal>SKIP LOCKED</literal>, any selected rows that cannot be
    immediately locked are skipped.  Skipping locked rows provides an
    inconsistent view of the data, so this is not suitable for general purpose
    work, but can be used to avoid lock contention with multiple consumers
    accessing a queue-like table.
    Note that <literal>NOWAIT</literal> and <literal>SKIP LOCKED</literal> apply only
    to the row-level lock(s) &mdash; the required <literal>ROW SHARE</literal>
    table-level lock is still taken in the ordinary way (see
    <xref linkend="mvcc"/>).  You can use
    <link linkend="sql-lock"><command>LOCK</command></link>
    with the <literal>NOWAIT</literal> option first,
    if you need to acquire the table-level lock without waiting.
   </para>

   <para>
    If specific tables are named in a locking clause,
    then only rows coming from those tables are locked; any other
    tables used in the <command>SELECT</command> are simply read as
    usual.  A locking
    clause without a table list affects all tables used in the statement.
    If a locking clause is
    applied to a view or sub-query, it affects all tables used in
    the view or sub-query.
    However, these clauses
    do not apply to <literal>WITH</literal> queries referenced by the primary query.
    If you want row locking to occur within a <literal>WITH</literal> query, specify
    a locking clause within the <literal>WITH</literal> query.
   </para>

   <para>
    Multiple locking
    clauses can be written if it is necessary to specify different locking
    behavior for different tables.  If the same table is mentioned (or
    implicitly affected) by more than one locking clause,
    then it is processed as if it was only specified by the strongest one.
    Similarly, a table is processed
    as <literal>NOWAIT</literal> if that is specified in any of the clauses
    affecting it.  Otherwise, it is processed
    as <literal>SKIP LOCKED</literal> if that is specified in any of the
    clauses affecting it.
   </para>

   <para>
    The locking clauses cannot be
    used in contexts where returned rows cannot be clearly identified with
    individual table rows; for example they cannot be used with aggregation.
   </para>

   <para>
    When a locking clause
    appears at the top level of a <command>SELECT</command> query, the rows that
    are locked are exactly those that are returned by the query; in the
    case of a join query, the rows locked are those that contribute to
    returned join rows.  In addition, rows that satisfied the query
    conditions as of the query snapshot will be locked, although they
    will not be returned if they were updated after the snapshot
    and no longer satisfy the query conditions.  If a
    <literal>LIMIT</literal> is used, locking stops
    once enough rows have been returned to satisfy the limit (but note that
    rows skipped over by <literal>OFFSET</literal> will get locked).  Similarly,
    if a locking clause
    is used in a cursor's query, only rows actually fetched or stepped past
    by the cursor will be locked.
   </para>

   <para>
    When a locking clause
    appears in a sub-<command>SELECT</command>, the rows locked are those
    returned to the outer query by the sub-query.  This might involve
    fewer rows than inspection of the sub-query alone would suggest,
    since conditions from the outer query might be used to optimize
    execution of the sub-query.  For example,
<programlisting>
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
</programlisting>
    will lock only rows having <literal>col1 = 5</literal>, even though that
    condition is not textually within the sub-query.
   </para>

  <para>
   Previous releases failed to preserve a lock which is upgraded by a later
   savepoint.  For example, this code:
<programlisting>
BEGIN;
SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
SAVEPOINT s;
UPDATE mytable SET ... WHERE key = 1;
ROLLBACK TO s;
</programlisting>
   would fail to preserve the <literal>FOR UPDATE</literal> lock after the
   <command>ROLLBACK TO</command>.  This has been fixed in release 9.3.
  </para>

  <caution>
   <para>
    It is possible for a <command>SELECT</command> command running at the <literal>READ
    COMMITTED</literal> transaction isolation level and using <literal>ORDER
    BY</literal> and a locking clause to return rows out of
    order.  This is because <literal>ORDER BY</literal> is applied first.
    The command sorts the result, but might then block trying to obtain a lock
    on one or more of the rows.  Once the <literal>SELECT</literal> unblocks, some
    of the ordering column values might have been modified, leading to those
    rows appearing to be out of order (though they are in order in terms
    of the original column values).  This can be worked around at need by
    placing the <literal>FOR UPDATE/SHARE</literal> clause in a sub-query,
    for example
<programlisting>
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
</programlisting>
    Note that this will result in locking all rows of <structname>mytable</structname>,
    whereas <literal>FOR UPDATE</literal> at the top level would lock only the
    actually returned rows.  This can make for a significant performance
    difference, particularly if the <literal>ORDER BY</literal> is combined with
    <literal>LIMIT</literal> or other restrictions.  So this technique is recommended
    only if concurrent updates of the ordering columns are expected and a
    strictly sorted result is required.
   </para>

   <para>
    At the <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal>
    transaction isolation level this would cause a serialization failure (with
    a <literal>SQLSTATE</literal> of <literal>'40001'</literal>), so there is
    no possibility of receiving rows out of order under these isolation levels.
   </para>
  </caution>
  </refsect2>

  <refsect2 id="sql-table">
   <title><literal>TABLE</literal> Command</title>

   <para>
    The command
<programlisting>
TABLE <replaceable class="parameter">name</replaceable>
</programlisting>
    is equivalent to
<programlisting>
SELECT * FROM <replaceable class="parameter">name</replaceable>
</programlisting>
    It can be used as a top-level command or as a space-saving syntax
    variant in parts of complex queries. Only the <literal>WITH</literal>,
    <literal>UNION</literal>, <literal>INTERSECT</literal>, <literal>EXCEPT</literal>,
    <literal>ORDER BY</literal>, <literal>LIMIT</literal>, <literal>OFFSET</literal>,
    <literal>FETCH</literal> and <literal>FOR</literal> locking clauses can be used
    with <command>TABLE</command>; the <literal>WHERE</literal> clause and any form of
    aggregation cannot
    be used.
   </para>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To join the table <literal>films</literal> with the table
   <literal>distributors</literal>:

<programlisting>
SELECT f.title, f.did, d.name, f.date_prod, f.kind
    FROM distributors d, films f
    WHERE f.did = d.did

       title       | did |     name     | date_prod  |   kind
-------------------+-----+--------------+------------+----------
 The Third Man     | 101 | British Lion | 1949-12-23 | Drama
 The African Queen | 101 | British Lion | 1951-08-11 | Romantic
 ...
</programlisting>
  </para>

  <para>
   To sum the column <literal>len</literal> of all films and group
   the results by <literal>kind</literal>:

<programlisting>
SELECT kind, sum(len) AS total FROM films GROUP BY kind;

   kind   | total
----------+-------
 Action   | 07:34
 Comedy   | 02:58
 Drama    | 14:28
 Musical  | 06:42
 Romantic | 04:38
</programlisting>
  </para>

  <para>
   To sum the column <literal>len</literal> of all films, group
   the results by <literal>kind</literal> and show those group totals
   that are less than 5 hours:

<programlisting>
SELECT kind, sum(len) AS total
    FROM films
    GROUP BY kind
    HAVING sum(len) &lt; interval '5 hours';

   kind   | total
----------+-------
 Comedy   | 02:58
 Romantic | 04:38
</programlisting>
  </para>

  <para>
   The following two examples are identical ways of sorting the individual
   results according to the contents of the second column
   (<literal>name</literal>):

<programlisting>
SELECT * FROM distributors ORDER BY name;
SELECT * FROM distributors ORDER BY 2;

 did |       name
-----+------------------
 109 | 20th Century Fox
 110 | Bavaria Atelier
 101 | British Lion
 107 | Columbia
 102 | Jean Luc Godard
 113 | Luso films
 104 | Mosfilm
 103 | Paramount
 106 | Toho
 105 | United Artists
 111 | Walt Disney
 112 | Warner Bros.
 108 | Westward
</programlisting>
  </para>

  <para>
   The next example shows how to obtain the union of the tables
   <literal>distributors</literal> and
   <literal>actors</literal>, restricting the results to those that begin
   with the letter W in each table.  Only distinct rows are wanted, so the
   key word <literal>ALL</literal> is omitted.

<programlisting>
distributors:               actors:
 did |     name              id |     name
-----+--------------        ----+----------------
 108 | Westward               1 | Woody Allen
 111 | Walt Disney            2 | Warren Beatty
 112 | Warner Bros.           3 | Walter Matthau
 ...                         ...

SELECT distributors.name
    FROM distributors
    WHERE distributors.name LIKE 'W%'
UNION
SELECT actors.name
    FROM actors
    WHERE actors.name LIKE 'W%';

      name
----------------
 Walt Disney
 Walter Matthau
 Warner Bros.
 Warren Beatty
 Westward
 Woody Allen
</programlisting>
  </para>

  <para>
   This example shows how to use a function in the <literal>FROM</literal>
   clause, both with and without a column definition list:

<programlisting>
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
    SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;

SELECT * FROM distributors(111);
 did |    name
-----+-------------
 111 | Walt Disney

CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
    SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;

SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
 f1  |     f2
-----+-------------
 111 | Walt Disney
</programlisting>
  </para>

  <para>
   Here is an example of a function with an ordinality column added:

<programlisting>
SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY;
 unnest | ordinality
--------+----------
 a      |        1
 b      |        2
 c      |        3
 d      |        4
 e      |        5
 f      |        6
(6 rows)
</programlisting>
  </para>

  <para>
   This example shows how to use a simple <literal>WITH</literal> clause:

<programlisting>
WITH t AS (
    SELECT random() as x FROM generate_series(1, 3)
  )
SELECT * FROM t
UNION ALL
SELECT * FROM t

         x          
--------------------
  0.534150459803641
  0.520092216785997
 0.0735620250925422
  0.534150459803641
  0.520092216785997
 0.0735620250925422
</programlisting>

   Notice that the <literal>WITH</literal> query was evaluated only once,
   so that we got two sets of the same three random values.
  </para>

  <para>
   This example uses <literal>WITH RECURSIVE</literal> to find all
   subordinates (direct or indirect) of the employee Mary, and their
   level of indirectness, from a table that shows only direct
   subordinates:

<programlisting>
WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
    SELECT 1, employee_name, manager_name
    FROM employee
    WHERE manager_name = 'Mary'
  UNION ALL
    SELECT er.distance + 1, e.employee_name, e.manager_name
    FROM employee_recursive er, employee e
    WHERE er.employee_name = e.manager_name
  )
SELECT distance, employee_name FROM employee_recursive;
</programlisting>

   Notice the typical form of recursive queries:
   an initial condition, followed by <literal>UNION</literal>,
   followed by the recursive part of the query. Be sure that the
   recursive part of the query will eventually return no tuples, or
   else the query will loop indefinitely.  (See <xref linkend="queries-with"/>
   for more examples.)
  </para>

  <para>
   This example uses <literal>LATERAL</literal> to apply a set-returning function
   <function>get_product_names()</function> for each row of the
   <structname>manufacturers</structname> table:

<programlisting>
SELECT m.name AS mname, pname
FROM manufacturers m, LATERAL get_product_names(m.id) pname;
</programlisting>

    Manufacturers not currently having any products would not appear in the
    result, since it is an inner join.  If we wished to include the names of
    such manufacturers in the result, we could do:

<programlisting>
SELECT m.name AS mname, pname
FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   Of course, the <command>SELECT</command> statement is compatible
   with the SQL standard.  But there are some extensions and some
   missing features.
  </para>

  <refsect2>
   <title>Omitted <literal>FROM</literal> Clauses</title>

   <para>
    <productname>PostgreSQL</productname> allows one to omit the
    <literal>FROM</literal> clause.  It has a straightforward use to
    compute the results of simple expressions:
<programlisting>
SELECT 2+2;

 ?column?
----------
        4
</programlisting>
    Some other <acronym>SQL</acronym> databases cannot do this except
    by introducing a dummy one-row table from which to do the
    <command>SELECT</command>.
   </para>
  </refsect2>

  <refsect2>
   <title>Empty <literal>SELECT</literal> Lists</title>

   <para>
    The list of output expressions after <literal>SELECT</literal> can be
    empty, producing a zero-column result table.
    This is not valid syntax according to the SQL standard.
    <productname>PostgreSQL</productname> allows it to be consistent with
    allowing zero-column tables.
    However, an empty list is not allowed when <literal>DISTINCT</literal> is used.
   </para>
  </refsect2>

  <refsect2>
   <title>Omitting the <literal>AS</literal> Key Word</title>

   <para>
    In the SQL standard, the optional key word <literal>AS</literal> can be
    omitted before an output column name whenever the new column name
    is a valid column name (that is, not the same as any reserved
    keyword).  <productname>PostgreSQL</productname> is slightly more
    restrictive: <literal>AS</literal> is required if the new column name
    matches any keyword at all, reserved or not.  Recommended practice is
    to use <literal>AS</literal> or double-quote output column names, to prevent
    any possible conflict against future keyword additions.
   </para>

   <para>
    In <literal>FROM</literal> items, both the standard and
    <productname>PostgreSQL</productname> allow <literal>AS</literal> to
    be omitted before an alias that is an unreserved keyword.  But
    this is impractical for output column names, because of syntactic
    ambiguities.
   </para>
  </refsect2>

  <refsect2>
   <title><literal>ONLY</literal> and Inheritance</title>

   <para>
    The SQL standard requires parentheses around the table name when
    writing <literal>ONLY</literal>, for example <literal>SELECT * FROM ONLY
    (tab1), ONLY (tab2) WHERE ...</literal>.  <productname>PostgreSQL</productname>
    considers these parentheses to be optional.
   </para>

   <para>
    <productname>PostgreSQL</productname> allows a trailing <literal>*</literal> to be written to
    explicitly specify the non-<literal>ONLY</literal> behavior of including
    child tables.  The standard does not allow this.
   </para>

   <para>
    (These points apply equally to all SQL commands supporting the
    <literal>ONLY</literal> option.)
   </para>
  </refsect2>

  <refsect2>
   <title><literal>TABLESAMPLE</literal> Clause Restrictions</title>

   <para>
    The <literal>TABLESAMPLE</literal> clause is currently accepted only on
    regular tables and materialized views.  According to the SQL standard
    it should be possible to apply it to any <literal>FROM</literal> item.
   </para>
  </refsect2>

  <refsect2>
   <title>Function Calls in <literal>FROM</literal></title>

   <para>
    <productname>PostgreSQL</productname> allows a function call to be
    written directly as a member of the <literal>FROM</literal> list.  In the SQL
    standard it would be necessary to wrap such a function call in a
    sub-<command>SELECT</command>; that is, the syntax
    <literal>FROM <replaceable>func</replaceable>(...) <replaceable>alias</replaceable></literal>
    is approximately equivalent to
    <literal>FROM LATERAL (SELECT <replaceable>func</replaceable>(...)) <replaceable>alias</replaceable></literal>.
    Note that <literal>LATERAL</literal> is considered to be implicit; this is
    because the standard requires <literal>LATERAL</literal> semantics for an
    <literal>UNNEST()</literal> item in <literal>FROM</literal>.
    <productname>PostgreSQL</productname> treats <literal>UNNEST()</literal> the
    same as other set-returning functions.
   </para>
  </refsect2>

  <refsect2>
   <title>Namespace Available to <literal>GROUP BY</literal> and <literal>ORDER BY</literal></title>

   <para>
    In the SQL-92 standard, an <literal>ORDER BY</literal> clause can
    only use output column names or numbers, while a <literal>GROUP
    BY</literal> clause can only use expressions based on input column
    names.  <productname>PostgreSQL</productname> extends each of
    these clauses to allow the other choice as well (but it uses the
    standard's interpretation if there is ambiguity).
    <productname>PostgreSQL</productname> also allows both clauses to
    specify arbitrary expressions.  Note that names appearing in an
    expression will always be taken as input-column names, not as
    output-column names.
   </para>

   <para>
    SQL:1999 and later use a slightly different definition which is not
    entirely upward compatible with SQL-92.
    In most cases, however, <productname>PostgreSQL</productname>
    will interpret an <literal>ORDER BY</literal> or <literal>GROUP
    BY</literal> expression the same way SQL:1999 does.
   </para>
  </refsect2>

  <refsect2>
   <title>Functional Dependencies</title>

   <para>
    <productname>PostgreSQL</productname> recognizes functional dependency
    (allowing columns to be omitted from <literal>GROUP BY</literal>) only when
    a table's primary key is included in the <literal>GROUP BY</literal> list.
    The SQL standard specifies additional conditions that should be
    recognized.
   </para>
  </refsect2>

  <refsect2>
   <title><literal>LIMIT</literal> and <literal>OFFSET</literal></title>

   <para>
    The clauses <literal>LIMIT</literal> and <literal>OFFSET</literal>
    are <productname>PostgreSQL</productname>-specific syntax, also
    used by <productname>MySQL</productname>.  The SQL:2008 standard
    has introduced the clauses <literal>OFFSET ... FETCH {FIRST|NEXT}
    ...</literal> for the same functionality, as shown above
    in <xref linkend="sql-limit"/>.  This
    syntax is also used by <productname>IBM DB2</productname>.
    (Applications written for <productname>Oracle</productname>
    frequently use a workaround involving the automatically
    generated <literal>rownum</literal> column, which is not available in
    PostgreSQL, to implement the effects of these clauses.)
   </para>
  </refsect2>

  <refsect2>
   <title><literal>FOR NO KEY UPDATE</literal>, <literal>FOR UPDATE</literal>, <literal>FOR SHARE</literal>, <literal>FOR KEY SHARE</literal></title>

   <para>
    Although <literal>FOR UPDATE</literal> appears in the SQL standard, the
    standard allows it only as an option of <command>DECLARE CURSOR</command>.
    <productname>PostgreSQL</productname> allows it in any <command>SELECT</command>
    query as well as in sub-<command>SELECT</command>s, but this is an extension.
    The <literal>FOR NO KEY UPDATE</literal>, <literal>FOR SHARE</literal> and
    <literal>FOR KEY SHARE</literal> variants, as well as the <literal>NOWAIT</literal>
    and <literal>SKIP LOCKED</literal> options, do not appear in the
    standard.
   </para>
  </refsect2>

  <refsect2>
   <title>Data-Modifying Statements in <literal>WITH</literal></title>

   <para>
    <productname>PostgreSQL</productname> allows <command>INSERT</command>,
    <command>UPDATE</command>, and <command>DELETE</command> to be used as <literal>WITH</literal>
    queries.  This is not found in the SQL standard.
   </para>
  </refsect2>

  <refsect2>
   <title>Nonstandard Clauses</title>

   <para>
    <literal>DISTINCT ON ( ... )</literal> is an extension of the
    SQL standard.
   </para>

   <para>
    <literal>ROWS FROM( ... )</literal> is an extension of the SQL standard.
   </para>

   <para>
    The <literal>MATERIALIZED</literal> and <literal>NOT
    MATERIALIZED</literal> options of <literal>WITH</literal> are extensions
    of the SQL standard.
   </para>
  </refsect2>

 </refsect1>
</refentry>