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diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml new file mode 100644 index 0000000..982835d --- /dev/null +++ b/doc/src/sgml/ref/select.sgml @@ -0,0 +1,2145 @@ +<!-- +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 <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> [, ...] ) ] + +<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> ) + +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 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 + <xref linkend="sql-values"/> 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> [, ...] )</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> + </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 <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. 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 — 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 <</literal> and + <literal>DESC</literal> is usually equivalent to <literal>USING ></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. + <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 — 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) — the required <literal>ROW SHARE</literal> + table-level lock is still taken in the ordinary way (see + <xref linkend="mvcc"/>). You can use + <xref linkend="sql-lock"/> + 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) < 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> + + <para> + Note that if a <literal>FROM</literal> clause is not specified, + the query cannot reference any database tables. For example, the + following query is invalid: +<programlisting> +SELECT distributors.* WHERE distributors.name = 'Westward'; +</programlisting><productname>PostgreSQL</productname> releases prior to + 8.1 would accept queries of this form, and add an implicit entry + to the query's <literal>FROM</literal> clause for each table + referenced by the query. This is no longer allowed. + </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> |