Mysql案例6_求平均薪资的等级最低的部门的部门名称

SELECT 
            t.*,s.grade
FROM 
            (
                    SELECT 
                            e.departmentid, d.`name`,AVG(salary) avgsal
                    FROM 
                            employee e
                    INNER JOIN
                            department d
                    on 
                            e.departmentid=d.id
                    GROUP BY 
                            e.departmentid,d.`name`  ) t
INNER JOIN
            salgrade s 
ON
            t.avgsal BETWEEN s.losal AND s.hisal
WHERE
            grade = (SELECT 
                               MIN(t.grade) mingra
                      FROM (
                                                SELECT 
                                                    t.*,s.grade
                                                FROM 
                                                        (
                                                                    SELECT 
                                                                            e.departmentid, d.`name`,AVG(salary) avgsal
                                                                    FROM 
                                                                            employee e
                                                                    INNER JOIN
                                                                            department d
                                                                    on 
                                                                            e.departmentid=d.id
                                                                    GROUP BY 
                                                                            e.departmentid,d.`name`) t
                                                    INNER JOIN
                                                            salgrade s
                                                    ON
                                                            t.avgsal BETWEEN s.losal AND s.hisal) t
                                                )