mysql聚合函数---总体聚合、总体累加、分组聚合、分组累加
MySQL从版本8.0开始，才支持窗口函数，所以之前的版本分组累加需要构造sql语句来实现。
数据：select * from emp;
一、mysql总体聚合函数
min()、max()、count()、sum()、avg()
select count(ename) ,max(sal),min(sal),sum(sal),round(avg(sal),2) from emp;二、mysql总体累加/总体累计数量
#累计求和/累计数量select @s :=0 ,@rank := 0;select *,@s := @s+sal , @rank := @rank+1 from emp order by sal;
三、分组聚合
select deptno,count(sal),sum(sal),round(avg(sal),2) from empgroup by deptnoorder by avg(sal);
select deptno,count(sal) from empgroup by deptno having deptno = 10 or deptno = 30 #只要10、30部门的数据order by count(sal) desc;
select deptno,count(sal) from emp #计算各个部门sal>=2000的员工个数where sal>=2000group by deptnoorder by count(sal) desc;
四、分组累加/累计计数
select empno,deptno,sal ,(select sum(sal) from emp where deptno = e.deptno and empno<=e.empno ) as g_sumfrom emp as e order by deptno ,empno;
select empno,deptno,sal ,(select sum(sal) from emp where deptno = e.deptno and sal<=e.sal ) as g_sum,(select count(sal) from emp where deptno = e.deptno and sal<=e.sal order by sal) as g_countfrom emp as e order by deptno ,sal;