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diff --git a/doc/src/sgml/html/queries-order.html b/doc/src/sgml/html/queries-order.html new file mode 100644 index 0000000..a81076e --- /dev/null +++ b/doc/src/sgml/html/queries-order.html @@ -0,0 +1,76 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>7.5. Sorting Rows</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="queries-union.html" title="7.4. Combining Queries" /><link rel="next" href="queries-limit.html" title="7.6. LIMIT and OFFSET" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">7.5. Sorting Rows</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="queries-union.html" title="7.4. Combining Queries">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="queries.html" title="Chapter 7. Queries">Up</a></td><th width="60%" align="center">Chapter 7. Queries</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="queries-limit.html" title="7.6. LIMIT and OFFSET">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="QUERIES-ORDER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">7.5. Sorting Rows</h2></div></div></div><a id="id-1.5.6.9.2" class="indexterm"></a><a id="id-1.5.6.9.3" class="indexterm"></a><p> + After a query has produced an output table (after the select list + has been processed) it can optionally be sorted. If sorting is not + chosen, the rows will be returned in an unspecified order. The actual + order in that case will depend on the scan and join plan types and + the order on disk, but it must not be relied on. A particular + output ordering can only be guaranteed if the sort step is explicitly + chosen. + </p><p> + The <code class="literal">ORDER BY</code> clause specifies the sort order: +</p><pre class="synopsis"> +SELECT <em class="replaceable"><code>select_list</code></em> + FROM <em class="replaceable"><code>table_expression</code></em> + ORDER BY <em class="replaceable"><code>sort_expression1</code></em> [<span class="optional">ASC | DESC</span>] [<span class="optional">NULLS { FIRST | LAST }</span>] + [<span class="optional">, <em class="replaceable"><code>sort_expression2</code></em> [<span class="optional">ASC | DESC</span>] [<span class="optional">NULLS { FIRST | LAST }</span>] ...</span>] +</pre><p> + The sort expression(s) can be any expression that would be valid in the + query's select list. An example is: +</p><pre class="programlisting"> +SELECT a, b FROM table1 ORDER BY a + b, c; +</pre><p> + When more than one expression is specified, + the later values are used to sort rows that are equal according to the + earlier values. Each expression can be followed by an optional + <code class="literal">ASC</code> or <code class="literal">DESC</code> keyword to set the sort direction to + ascending or descending. <code class="literal">ASC</code> order is the default. + Ascending order puts smaller values first, where + <span class="quote">“<span class="quote">smaller</span>”</span> is defined in terms of the + <code class="literal"><</code> operator. Similarly, descending order is + determined with the <code class="literal">></code> operator. + <a href="#ftn.id-1.5.6.9.5.10" class="footnote"><sup class="footnote" id="id-1.5.6.9.5.10">[5]</sup></a> + </p><p> + The <code class="literal">NULLS FIRST</code> and <code class="literal">NULLS LAST</code> options can be + used to determine whether nulls appear before or after non-null values + in the sort ordering. By default, null values sort as if larger than any + non-null value; that is, <code class="literal">NULLS FIRST</code> is the default for + <code class="literal">DESC</code> order, and <code class="literal">NULLS LAST</code> otherwise. + </p><p> + Note that the ordering options are considered independently for each + sort column. For example <code class="literal">ORDER BY x, y DESC</code> means + <code class="literal">ORDER BY x ASC, y DESC</code>, which is not the same as + <code class="literal">ORDER BY x DESC, y DESC</code>. + </p><p> + A <em class="replaceable"><code>sort_expression</code></em> can also be the column label or number + of an output column, as in: +</p><pre class="programlisting"> +SELECT a + b AS sum, c FROM table1 ORDER BY sum; +SELECT a, max(b) FROM table1 GROUP BY a ORDER BY 1; +</pre><p> + both of which sort by the first output column. Note that an output + column name has to stand alone, that is, it cannot be used in an expression + — for example, this is <span class="emphasis"><em>not</em></span> correct: +</p><pre class="programlisting"> +SELECT a + b AS sum, c FROM table1 ORDER BY sum + c; -- wrong +</pre><p> + This restriction is made to reduce ambiguity. There is still + ambiguity if an <code class="literal">ORDER BY</code> item is a simple name that + could match either an output column name or a column from the table + expression. The output column is used in such cases. This would + only cause confusion if you use <code class="literal">AS</code> to rename an output + column to match some other table column's name. + </p><p> + <code class="literal">ORDER BY</code> can be applied to the result of a + <code class="literal">UNION</code>, <code class="literal">INTERSECT</code>, or <code class="literal">EXCEPT</code> + combination, but in this case it is only permitted to sort by + output column names or numbers, not by expressions. + </p><div class="footnotes"><br /><hr style="width:100; text-align:left;margin-left: 0" /><div id="ftn.id-1.5.6.9.5.10" class="footnote"><p><a href="#id-1.5.6.9.5.10" class="para"><sup class="para">[5] </sup></a> + Actually, <span class="productname">PostgreSQL</span> uses the <em class="firstterm">default B-tree + operator class</em> for the expression's data type to determine the sort + ordering for <code class="literal">ASC</code> and <code class="literal">DESC</code>. Conventionally, + data types will be set up so that the <code class="literal"><</code> and + <code class="literal">></code> operators correspond to this sort ordering, + but a user-defined data type's designer could choose to do something + different. + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="queries-union.html" title="7.4. Combining Queries">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="queries.html" title="Chapter 7. Queries">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="queries-limit.html" title="7.6. LIMIT and OFFSET">Next</a></td></tr><tr><td width="40%" align="left" valign="top">7.4. Combining Queries </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 7.6. <code xmlns="http://www.w3.org/1999/xhtml" class="literal">LIMIT</code> and <code xmlns="http://www.w3.org/1999/xhtml" class="literal">OFFSET</code></td></tr></table></div></body></html>
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