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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>

</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>