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mysql语句的执行顺序问题 是select 先执行还是group by 先执行？是select 先执行 还是 having 子句先执行？？ mysql> select (@a :=empid) a ,heyf_t10.* from heyf_t10 ; +---+-------+--------+--------+ | a | empid | deptid | salary | +---+-------+--------+--------+ | 1 | 1 | 10 | 5500 | | 2 | 2 | 10 | 4500 | | 3 | 3 | 20 | 1900 | | 4 | 4 | 20 | 4800 | | 5 | 5 | 40 | 6500 | | 6 | 6 | 40 | 14500 | | 7 | 7 | 40 | 44500 | | 8 | 8 | 50 | 6500 | | 9 | 9 | 50 | 7500 | +---+-------+--------+--------+ 9 rows in set mysql> select (@a :=empid) a ,heyf_t10.* from heyf_t10 having @a =5; Empty set 第二个查询 条件是 having @a=5 的时候 结果集为什么是空的呢？？ --------------------------------------------------------------- mysql> select (@a :=empid) a ,heyf_t10.* from heyf_t10 having empid =5; +---+-------+--------+--------+ | a | empid | deptid | salary | +---+-------+--------+--------+ | 5 | 5 | 40 | 6500 | +---+-------+--------+--------+ 1 row in set 为什么 当条件 使用having empid=5 的时候能够正常返回数据记录 总结 不能在group by， having，ORDER BY子句后边使用 用户变量 bitsCN.com
