Mysql 练习题10道(1-10题)

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练习

取得每个部门最高薪水的人员名称

  • 第一步：
    取得每个部门最高薪水（按照部门编号分组，找出每一组最大值）
    select max(sal) maxsal from emp group by deptno;
+---------+
| maxsal  |
+---------+
| 3000.00 |
| 2850.00 |
| 5000.00 |
+---------+
  • 第二步：
    将以上结果当做一张临时表和emp连接
    select e.ename,e.sal,e.deptno from emp e join (select max(sal) maxsal from emp group by deptno) t on e.sal = t.maxsal;
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| BLAKE | 2850.00 |     30 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+

哪些人的薪水在部门的平均薪水之上

第一步：取得每个部门的平均薪水：
select deptno,avg(sal) avgsal from emp group by deptno;

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|     10 | 2916.666667 |
+--------+-------------+

第二步：将以上结果当做一张临时表和emp连接：
select e.ename,e.sal,e.deptno,t.* from emp e join (select deptno,avg(sal) avgsal from emp group by deptno) t on e.sal>t.avgsal and e.deptno=t.deptno;

+-------+---------+--------+--------+-------------+
| ename | sal     | deptno | deptno | avgsal      |
+-------+---------+--------+--------+-------------+
| ALLEN | 1600.00 |     30 |     30 | 1566.666667 |
| JONES | 2975.00 |     20 |     20 | 2175.000000 |
| BLAKE | 2850.00 |     30 |     30 | 1566.666667 |
| SCOTT | 3000.00 |     20 |     20 | 2175.000000 |
| KING  | 5000.00 |     10 |     10 | 2916.666667 |
| FORD  | 3000.00 |     20 |     20 | 2175.000000 |
+-------+---------+--------+--------+-------------+

取得部门中（所有人的）平均的薪水等级

第一步：找出每一个人的薪水等级
select t.grade,e.ename,e.sal,e.deptno from emp e join salgrade t on e.sal between t.losal and t.hisal;

+-------+--------+---------+--------+
| grade | ename  | sal     | deptno |
+-------+--------+---------+--------+
|     1 | SMITH  |  800.00 |     20 |
|     3 | ALLEN  | 1600.00 |     30 |
|     2 | WARD   | 1250.00 |     30 |
|     4 | JONES  | 2975.00 |     20 |
|     2 | MARTIN | 1250.00 |     30 |
|     4 | BLAKE  | 2850.00 |     30 |
|     4 | CLARK  | 2450.00 |     10 |
|     4 | SCOTT  | 3000.00 |     20 |
|     5 | KING   | 5000.00 |     10 |
|     3 | TURNER | 1500.00 |     30 |
|     1 | ADAMS  | 1100.00 |     20 |
|     1 | JAMES  |  950.00 |     30 |
|     4 | FORD   | 3000.00 |     20 |
|     2 | MILLER | 1300.00 |     10 |
+-------+--------+---------+--------+

第二步：将以上结果按照dept分组获得等级平均值
select e.deptno,avg(t.grade) from emp e join salgrade t on e.sal between t.losal and t.hisal group by deptno;

+--------+--------------+
| deptno | avg(t.grade) |
+--------+--------------+
|     20 |       2.8000 |
|     30 |       2.5000 |
|     10 |       3.6667 |
+--------+--------------+

不准用聚合函数max，取得最高薪水

第一种方法：sal降序+limit
select ename,sal,deptno from emp order by sal desc limit 0,1;

+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| KING  | 5000.00 |     10 |
+-------+---------+--------+

第二种方法：表的自连接
select ename,sal from emp where sal not in(select distinct a.sal from emp a join emp b on a.sal < b.sal);

+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| KING  | 5000.00 |     10 |
+-------+---------+--------+

取得平均薪水最高的部门的部门编号

方法一：
第一步：取得各部门平均薪水
select deptno,avg(sal) avgsal from emp group by deptno;

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|     10 | 2916.666667 |
+--------+-------------+

第二步：将以上结果进行自连接取得部门编号
select deptno,avg(sal) avgsal from emp group by deptno order by avgsal limit 1;

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
+--------+-------------+

方法二：
第一步：取得各部门平均薪水
select deptno,avg(sal) avgsal from emp group by deptno;

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     20 | 2175.000000 |
|     30 | 1566.666667 |
|     10 | 2916.666667 |
+--------+-------------+

第二步：获得平均薪水最大值并用having进行二次筛选
select deptno,avg(sal) avgsal from emp group by deptno having avgsal = (select max(t.avgsal) from (select deptno,avg(sal) avgsal from emp group by deptno) t);

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
+--------+-------------+

取得平均薪水最高的部门的部门名称

和上一题相似，只不过需要对结果再进行一次连接查询：
select d.dname,e.deptno,avg(e.sal) avgsal from emp e join dept d on d.deptno=e.deptno group by deptno order by avgsal desc limit 1;

+------------+--------+-------------+
| dname      | deptno | avgsal      |
+------------+--------+-------------+
| ACCOUNTING |     10 | 2916.666667 |
+------------+--------+-------------+

求平均薪水的等级最低的部门的部门名称

方法一：
第一步：找出每个部门的平均薪水
select deptno,avg(sal) as avgsal from emp group by deptno;

+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+

第二步：找出每个部门的平均薪水的等级
第一步的t表和salgrade表连接，条件：t.avgsal between s.losal and s.hisal
select t.*,s.grade from salgrade s join (select d.dname,avg(e.sal) as avgsal from emp e join dept d on e.deptno = d.deptno group by d.dname) t on t.avgsal between s.losal and s.hisal;

+------------+-------------+-------+
| dname      | avgsal      | grade |
+------------+-------------+-------+
| RESEARCH   | 2175.000000 |     4 |
| SALES      | 1566.666667 |     3 |
| ACCOUNTING | 2916.666667 |     4 |
+------------+-------------+-------+

第三步：加上where条件
select t.*,s.grade from salgrade s join (select d.dname,avg(e.sal) as avgsal from emp e join dept d on e.deptno = d.deptno group by d.dname) t on t.avgsal between s.losal and s.hisal where s.grade = (select grade from salgrade where (select avg(sal) avgsal from emp group by deptno order by avgsal asc limit 1) between losal and hisal);

+-------+-------------+-------+
| dname | avgsal      | grade |
+-------+-------------+-------+
| SALES | 1566.666667 |     3 |
+-------+-------------+-------+

取得比普通员工(员工代码没有在mgr 字段上出现的) 的最高薪水还要高的领导人姓名

第一步：取得普通员工的最高薪水
select max(sal) maxsal from emp where empno not in (select mgr from emp where mgr is not null);

+---------+
| maxsal  |
+---------+
| 1600.00 |
+---------+

第二步：取得比maxsal高的领导名字
select ename,sal from emp where sal>1600 and empno in (select mgr from emp where mgr is not null);

+-------+---------+
| ename | sal     |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING  | 5000.00 |
| FORD  | 3000.00 |
+-------+---------+

取得薪水最高的前五名员工

select ename,sal from emp order by sal desc limit 5;

+-------+---------+
| ename | sal     |
+-------+---------+
| KING  | 5000.00 |
| SCOTT | 3000.00 |
| FORD  | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
+-------+---------+

取得薪水最高的第六到第十名员工

select ename,sal from emp order by sal desc limit 5,5;

+--------+---------+
| ename  | sal     |
+--------+---------+
| CLARK  | 2450.00 |
| ALLEN  | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| WARD   | 1250.00 |
+--------+---------+

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