【数据库】MySQL聚合统计

王笃笃-CSDN博客https://blog.csdn.net/wangduduniubi?type=blog显示平均工资低于2000的部门和它的平均工资

mysql> select deptno,avg(sal) deptavg  from emp group by deptno;
+--------+-------------+
| deptno | deptavg     |
+--------+-------------+
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|     10 | 2916.666667 |
+--------+-------------+
3 rows in set (0.01 sec)

mysql> select deptno,avg(sal) deptavg  from emp group by deptno having deptavg<2000;
+--------+-------------+
| deptno | deptavg     |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+

having 对聚合后的数据进行条件筛选。

having 和 where 的区别

条件筛选的阶段不同

mysql> select deptno,job,avg(sal) myavg  from emp where ename != 'SMITH' group by deptno,job having myavg<2000;
+--------+----------+-------------+
| deptno | job      | myavg       |
+--------+----------+-------------+
|     30 | SALESMAN | 1400.000000 |
|     20 | CLERK    | 1100.000000 |
|     30 | CLERK    |  950.000000 |
|     10 | CLERK    | 1300.000000 |
+--------+----------+-------------+

where

对具体的任意列进行条件筛选

having

对分组聚合之后的结果进行条件筛选。

条件筛选的阶段不同

1

from emp

2

where ename != 'SMITH'

3

group by deptno,job

4

deptno,job,avg(sal) myavg

5

having myavg<2000;

中间筛选出来的和最终结果——>全部可以看做是逻辑上的表-->mysql 中一切皆表