<para>
      The <literal>LIKE</literal> operator does pattern matching and
      is explained in <xref linkend="functions-matching"/>.
     </para>
    </callout>
   </calloutlist>
   </para>

   <para>
    It is important to understand the interaction between aggregates and
    <acronym>SQL</acronym>'s <literal>WHERE</literal> and <literal>HAVING</literal> clauses.
    The fundamental difference between <literal>WHERE</literal> and
    <literal>HAVING</literal> is this: <literal>WHERE</literal> selects
    input rows before groups and aggregates are computed (thus, it controls
    which rows go into the aggregate computation), whereas
    <literal>HAVING</literal> selects group rows after groups and
    aggregates are computed.  Thus, the
    <literal>WHERE</literal> clause must not contain aggregate functions;
    it makes no sense to try to use an aggregate to determine which rows
    will be inputs to the aggregates.  On the other hand, the
    <literal>HAVING</literal> clause always contains aggregate functions.
    (Strictly speaking, you are allowed to write a <literal>HAVING</literal>
    clause that doesn't use aggregates, but it's seldom useful. The same
    condition could be used more efficiently at the <literal>WHERE</literal>
    stage.)
   </para>

   <para>
    In the previous example, we can apply the city name restriction in
    <literal>WHERE</literal>, since it needs no aggregate.  This is
    more efficient than adding the restriction to <literal>HAVING</literal>,
    because we avoid doing the grouping and aggregate calculations
    for all rows that fail the <literal>WHERE</literal> check.
   </para>

   <para>
    Another way to select the rows that go into an aggregate
    computation is to use <literal>FILTER</literal>, which is a
    per-aggregate option:

<programlisting>
SELECT city, count(*) FILTER (WHERE temp_lo &lt; 45), max(temp_lo)
    FROM weather
    GROUP BY city;
</programlisting>

<screen>
     city      | count | max
---------------+-------+-----
 Hayward       |     1 |  37
 San Francisco |     1 |  46
(2 rows)
</screen>

    <literal>FILTER</literal> is much like <literal>WHERE</literal>,
    except that it removes rows only from the input of the particular
    aggregate function that it is attached to.
    Here, the <literal>count</literal> aggregate counts only