-
mysql 中的
having也是条件筛选语句，跟where的含义一样，但是having只用于对group by分组的结果进行条件筛选，所以可以先看看 group by 分组。 -
having跟where其实是一个意思，只是having跟在group by后面，where跟在from后面。 -
这里我就在 group by 分组 测试数据基础上往下写了。
mysql> select name, count(*) as 数量, avg(age) as 平均年龄 from test group by name;
+------+--------+--------------+
| name | 数量 | 平均年龄 |
+------+--------+--------------+
| NULL | 1 | NULL |
| djy | 1 | 5.0000 |
| dzm | 1 | 20.0000 |
| xyq | 2 | 20.0000 |
+------+--------+--------------+
平均年龄 > 10 的才显示
mysql> select name, count(*) as 数量, avg(age) as 平均年龄 from test group by name having avg(age) > 10;
+------+--------+--------------+
| name | 数量 | 平均年龄 |
+------+--------+--------------+
| dzm | 1 | 20.0000 |
| xyq | 2 | 20.0000 |
+------+--------+--------------+
where 与 having 也是可以同时存在的，先通过 where 过滤在通过分组之后 having 过滤
mysql> select name, count(*) as 数量, avg(age) as 平均年龄 from test where name='dzm' group by name having avg(age) > 10;
+------+--------+--------------+
| name | 数量 | 平均年龄 |
+------+--------+--------------+
| dzm | 1 | 20.0000 |
+------+--------+--------------+
