Merge pull request #438 from TsinghuaLucky912/master

补充 query.sgml 遗漏

Author: ChenHuajun

postgresql/doc/src/sgml/query.sgml

@@ -1383,6 +1383,56 @@ ____________________________________________________________________________-->
    <para>
     在前面的例子里，我们可以在<literal>WHERE</literal>里应用城市名称限制，因为它不需要聚集。这样比放在<literal>HAVING</literal>里更加高效，因为可以避免那些未通过 <literal>WHERE</literal>检查的行参与到分组和聚集计算中。
    </para>
+
+<!--==========================orignal english content==========================
+   <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
+-&minus;-&minus;-&minus;-&minus;-+-&minus;-&minus;-&minus;-&minus;-+-&minus;-&minus;-
+ 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
+    rows with <literal>temp_lo</literal> below 45; but the
+    <literal>max</literal> aggregate is still applied to all rows,
+    so it still finds the reading of 46.
+   </para>
+____________________________________________________________________________-->
+
+   <para>
+    选择进入聚合计算的行的另一种方法是使用<literal>FILTER</literal>，这是一个针对每个聚合的选项：
+
+<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> 与 <literal>WHERE</literal> 非常相似，不同之处在于它仅从其所附加的特定聚合函数的输入中删除行。此处，<literal>count</literal> 聚合仅对 <literal>temp_lo</literal> 低于 45 的行进行计数；但 <literal>max</literal> 聚合仍然应用于所有行，因此它仍然找到读数 46。
+   </para>
   </sect1>