聚集函数的使用

select * from sc order by grade

select * from sc order by grade desc

select count(*) from stu

select count(distinct sno) from sc

—以什么为区分 group by
select cno,count(sno) as num from sc group by cno

—where 后不出现聚集函数

—having 和group by 连用
select sno,avg(grade) from sc group by sno having avg(grade)>=90