分组函数的嵌套 目的：求部门平均工资的最大值
select max(avg(sal)) from emp group by deptno;GROUP BY 语句的增强
select deptno,job,sum(sal) from emp group by deptno,job order by deptno; select sum(sal) from emp group by deptno; select sum(sal) from emp;以上等于
select deptno,job,sum(sal) from emp group by rollup(deptno,job); group by rollup(a,b) 等价于： group by a,b + group by a + group by null
