sql group by, having

group by:分组

A B

1 3

1 5

1 7

select A,avg(B) from table group by A;

得到:

A avg(B)

1,5

select的元素中，若非聚合，则必须出现在group by的后面

所谓聚合函数，是count(),avg(),sum(),max(),min()等函数，将多个数据通过运算合成一个数据。

having:查询条件中若有聚合元素，则要用having判断，不能用where

select A,avgB(B) from table group by A having avg(B)>3;

select * from table a where a.salary>( select avg(salary) from table b where b.department=a.department group by department);

找出同一department中salary大于平均值的数据项