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<chapter id="sql-syntax">

<title>SQL Syntax</title>

unspecified order. In many cases this does not matter; for example,

<function>min</> produces the same result no matter what order it

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

such an aggregate, the optional <replaceable>order_by_clause</> can be

used to specify the desired ordering. The <replaceable>order_by_clause</>

has the same syntax as for a query-level <literal>ORDER BY</> clause, as

described in <xref linkend="queries-order">, except that its expressions

are always just expressions and cannot be output-column names or numbers.

For example:

<programlisting>

SELECT array_agg(a ORDER BY b DESC) FROM table;

</programlisting>

</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>

If <literal>DISTINCT</> is specified in addition to an

<replaceable>order_by_clause</>, then all the <literal>ORDER BY</>