Where and Having区别

Where和Having的异同

(1)where是查询返回结果之前进行过滤的

(2)having是查询返回结果之后，对结果进行过滤的

(3)在SQL中增加 HAVING 子句原因是，where关键字无法与聚合函数一起使用,having子句常跟group by一同使用，过滤分组后的数据

测试表

mysql> select * from t_order;+--------+---------+------------+------------+| emp_no | dept_no | from_date  | to_date    |+--------+---------+------------+------------+|  22744 | d006    | 1986-12-01 | 9999-01-01 ||  24007 | d005    | 1986-12-01 | 9999-01-01 ||  30970 | d005    | 1986-12-01 | 2017-03-29 ||  31112 | d002    | 1986-12-01 | 1993-12-10 ||  40983 | d005    | 1986-12-01 | 9999-01-01 ||   NULL | d008    | 1986-12-01 | 1992-05-27 ||  48317 | d008    | 1986-12-01 | 1989-01-11 ||  49667 | d007    | 1986-12-01 | 9999-01-01 ||  50449 | d005    | 1986-12-01 | 9999-01-01 ||  10004 | d004    | 1986-12-01 | 9999-01-01 |+--------+---------+------------+------------+10 rows in set (0.00 sec)

Where后面如果提前使用列emp_no的别名aaa进行过滤的话，会报错列不存在，这是因为where字句是先对表进行过滤，才开始查询结果的

mysql> select emp_no as aaa from t_order where emp_no=22744;+-------+| aaa   |+-------+| 22744 |+-------+1 row in set (0.00 sec)mysql> select emp_no as aaa from t_order where aaa=22744;ERROR 1054 (42S22): Unknown column 'aaa' in 'where clause'

Having后面可以跟列emp_no的原名或者别名aaa都可以，也可以跟group by，然后对分组后的聚合函数列进行筛选

mysql> select emp_no as aaa from t_order having emp_no=22744;+-------+| aaa   |+-------+| 22744 |+-------+1 row in set (0.01 sec)mysql> select emp_no as aaa from t_order having aaa=22744;+-------+| aaa   |+-------+| 22744 |+-------+1 row in set (0.00 sec)mysql> select dept_no,min(emp_no) aaa from t_order  group by dept_no having aaa=31112;+---------+-------+| dept_no | aaa   |+---------+-------+| d002    | 31112 |+---------+-------+1 row in set (0.00 sec)