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>Using Aggregate Functions</TITLE
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>Using Aggregate Functions</A
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><P
> Like most other query languages, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports
aggregate functions.
The current implementation of <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> aggregate functions have some limitations.
Specifically, while there are aggregates to compute
such functions as the <TT
CLASS="FUNCTION"
>count</TT
>, <TT
CLASS="FUNCTION"
>sum</TT
>,
<TT
CLASS="FUNCTION"
>avg</TT
> (average), <TT
CLASS="FUNCTION"
>max</TT
> (maximum) and
<TT
CLASS="FUNCTION"
>min</TT
> (minimum) over a set of instances, aggregates can only
appear in the target list of a query and not directly in the
qualification (the <I
CLASS="FIRSTTERM"
>where</I
> clause). As an example,
<PRE
CLASS="PROGRAMLISTING"
>SELECT max(temp_lo) FROM weather;</PRE
>
is allowed, while
<PRE
CLASS="PROGRAMLISTING"
>SELECT city FROM weather WHERE temp_lo = max(temp_lo);</PRE
>
is not. However, as is often the case the query can be restated to accomplish
the intended result; here by using a <I
CLASS="FIRSTTERM"
>subselect</I
>:
<PRE
CLASS="PROGRAMLISTING"
>SELECT city FROM weather WHERE temp_lo = (SELECT max(temp_lo) FROM weather);</PRE
></P
><P
> Aggregates may also have <I
CLASS="FIRSTTERM"
>group by</I
> clauses:
<PRE
CLASS="PROGRAMLISTING"
>SELECT city, max(temp_lo)
FROM weather
GROUP BY city;</PRE
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