组函数 嵌套 找出平均工资大于300的部门的平均工资的最高工资
select max(aaa.avg_sal),aaa.avg_name from(select avg(e.t_salary) as avg_sal,d.t_dname as avg_name,d.t_dnofrom t_dept d inner join t_employee e on e.d_id=d.t_id group by d.t_dname,d.t_dno having avg(e.t_salary)>300) aaagroup by aaa.avg_name
