一、聚合函数
1.定义：对表或视图的查询时，针对多行记录只返回一个值的函数。
2.用途：用于select语句，HAVING条件
二、5种聚合函数
1.SUM(n) 对列求和
select sum(sal) from emp;
select deptno,sum(sal) from emp group by deptno;
2.AVG(n) 对列求平均值
select avg(sal) from emp;
select deptno,avg(sal) from emp group by deptno;
3.MAX(n) 对列求最大值
select max(sal) from emp;
select deptno,max(sal) from emp group by deptno;
4.MIN(n) 对列求最小值
select min(sal) from emp;
select deptno,min(sal) from emp group by deptno;
5.COUNT(1) 记录行数
select COUNT(1) from emp;
select deptno,COUNT(1) from emp group by deptno;
select deptno,avg(sal),sum(sal)/COUNT(1) from emp group by deptno;
count(可空列)对null值不计数
select count(comm) from emp;
count(distinct 列)对null值不计数
select count(distinct empno),count(distinct deptno) from emp;
三、分组语句
语法：group by 列名,... having 条件
1.按部门分组，输出5种聚合函数，并按部门排序
select deptno,sum(sal),avg(sal),count(1),max(sal),min(sal)
from emp group by deptno order by deptno;
2.按部门分组，输出部门人数大于3的部门编号及5种聚合函数，并按部门排序
select deptno,sum(sal),avg(sal),count(1),max(sal),min(sal)
from emp group by deptno having count(1)>3 order by deptno;
3.按部门分组，输出部门人数大于等于3并且小于等于6
的部门编号及5种聚合函数，并按部门排序
select deptno,sum(sal),avg(sal),count(1),max(sal),min(sal)
from emp group by deptno having count(1)>=3 and count(1)<=6
order by deptno;
4.按部门分组，输出工资大于1000，部门人数大于等于3并且小于等于6
的部门编号及5种聚合函数，并按部门排序
select deptno,sum(sal),avg(sal),count(1),max(sal),min(sal)
from emp where sal>1000
group by deptno having count(1)>=3 and count(1)<=6
order by deptno;