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Fix subsection ordering (DISTINCT should be described before LIMIT).

This commit is contained in:
Tom Lane 2005-01-04 08:59:45 +00:00
parent 852b4ae5c2
commit a7abae4954
1 changed files with 40 additions and 40 deletions
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80
doc/src/sgml/ref/select.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.79 2005/01/04 03:58:16 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.80 2005/01/04 08:59:45 tgl Exp $
PostgreSQL documentation
-->
@ -695,6 +695,45 @@ SELECT name FROM distributors ORDER BY code;
was initialized.
</para>
</refsect2>
<refsect2 id="sql-distinct">
<title id="sql-distinct-title"><literal>DISTINCT</literal> Clause</title>
<para>
If <literal>DISTINCT</> is specified, all duplicate rows are
removed from the result set (one row is kept from each group of
duplicates). <literal>ALL</> specifies the opposite: all rows are
kept; that is the default.
</para>
<para>
<literal>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</> (see above). Note that the <quote>first
row</quote> of each set is unpredictable unless <literal>ORDER
BY</> 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</> 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</> expression(s) must match the leftmost
<literal>ORDER BY</> expression(s). The <literal>ORDER BY</> clause
will normally contain additional expression(s) that determine the
desired precedence of rows within each <literal>DISTINCT ON</> group.
</para>
</refsect2>
<refsect2 id="SQL-LIMIT">
<title id="sql-limit-title"><literal>LIMIT</literal> Clause</title>
@ -739,45 +778,6 @@ OFFSET <replaceable class="parameter">start</replaceable>
</para>
</refsect2>
<refsect2 id="sql-distinct">
<title id="sql-distinct-title"><literal>DISTINCT</literal> Clause</title>
<para>
If <literal>DISTINCT</> is specified, all duplicate rows are
removed from the result set (one row is kept from each group of
duplicates). <literal>ALL</> specifies the opposite: all rows are
kept; that is the default.
</para>
<para>
<literal>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</> (see above). Note that the <quote>first
row</quote> of each set is unpredictable unless <literal>ORDER
BY</> 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</> 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</> expression(s) must match the leftmost
<literal>ORDER BY</> expression(s). The <literal>ORDER BY</> clause
will normally contain additional expression(s) that determine the
desired precedence of rows within each <literal>DISTINCT ON</> group.
</para>
</refsect2>
<refsect2 id="SQL-FOR-UPDATE">
<title id="sql-for-update-title"><literal>FOR UPDATE</literal> Clause</title>