SQL语句下groupby,having的用法解析

group by

用来和聚合函数(cout, sum, max ,min, avg等)联合使用来得到一个或多个列的结果集合.

语法:

select column1,column2......column_n, aggregate_function(expression)

from tables

where predicates

group by column1,column2......column_n

例如: 查询每个学生选了几门课程及每个学生的平均分数

select id, count(course), avg(score)

from student

group by id

having

与group by联合使用,用来过滤由group by语句返回的记录值

例如:查询平均分大于80分的学生记录

select id, count(course), avg(score)

from student

group by id

having avg(score)>=80