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Add an example to clarify the use of ORDER BY in multiple-argument

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aggregates.  People seem to not get this right without help.
This commit is contained in:
Tom Lane
2010-08-04 15:27:57 +00:00
parent 145343534c
commit 72150db0c5
1 changed files with 17 additions and 3 deletions
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doc/src/sgml/syntax.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.148 2010/07/20 00:34:44 rhaas Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.149 2010/08/04 15:27:57 tgl Exp $ -->
<chapter id="sql-syntax"> <chapter id="sql-syntax">
<title>SQL Syntax</title> <title>SQL Syntax</title>
@ -1567,7 +1567,7 @@ sqrt(2)
unspecified order. In many cases this does not matter; for example, unspecified order. In many cases this does not matter; for example,
<function>min</> produces the same result no matter what order it <function>min</> produces the same result no matter what order it
receives the inputs in. However, some aggregate functions receives the inputs in. However, some aggregate functions
(such as <function>array_agg</> and <function>xmlagg</>) produce (such as <function>array_agg</> and <function>string_agg</>) produce
results that depend on the ordering of the input rows. When using results that depend on the ordering of the input rows. When using
such an aggregate, the optional <replaceable>order_by_clause</> can be such an aggregate, the optional <replaceable>order_by_clause</> can be
used to specify the desired ordering. The <replaceable>order_by_clause</> used to specify the desired ordering. The <replaceable>order_by_clause</>
@ -1575,12 +1575,26 @@ sqrt(2)
described in <xref linkend="queries-order">, except that its expressions described in <xref linkend="queries-order">, except that its expressions
are always just expressions and cannot be output-column names or numbers. are always just expressions and cannot be output-column names or numbers.
For example: For example:
<programlisting> <programlisting>
SELECT array_agg(a ORDER BY b DESC) FROM table; SELECT array_agg(a ORDER BY b DESC) FROM table;
</programlisting> </programlisting>
</para> </para>
<para>
When dealing with multiple-argument aggregate functions, note that the
<literal>ORDER BY</> clause goes after all the aggregate arguments.
For example, this:
<programlisting>
SELECT string_agg(a, ',' ORDER BY a) FROM table;
</programlisting>
not this:
<programlisting>
SELECT string_agg(a ORDER BY a, ',') FROM table; -- not what you want
</programlisting>
The latter syntax will be accepted, but <literal>','</> will be
treated as a (useless) sort key.
</para>
<para> <para>
If <literal>DISTINCT</> is specified in addition to an If <literal>DISTINCT</> is specified in addition to an
<replaceable>order_by_clause</>, then all the <literal>ORDER BY</> <replaceable>order_by_clause</>, then all the <literal>ORDER BY</>