- 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 | +------+--------+--------------+ ```
