动力节点 mysql 郭鑫 34道经典的面试题

DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
  `DEPTNO` int(2) NOT NULL COMMENT '部门编号',
  `DNAME` varchar(14) DEFAULT NULL COMMENT '部门名称',
  `LOC` varchar(13) DEFAULT NULL COMMENT '位置',
  PRIMARY KEY (`DEPTNO`)
) ENGINE=InnoDB;

INSERT INTO `dept` VALUES ('10', 'ACCOUNTING', 'NEW YORK');
INSERT INTO `dept` VALUES ('20', 'RESEARCH', 'DALLAS');
INSERT INTO `dept` VALUES ('30', 'SALES', 'CHICAGO');
INSERT INTO `dept` VALUES ('40', 'OPERATIONS', 'BOSTON');

 创建员工表

DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (
  `EMPNO` int(4) NOT NULL COMMENT '员工编号',
  `ENAME` varchar(10) DEFAULT NULL COMMENT '员工姓名',
  `JOB` varchar(9) DEFAULT NULL COMMENT '工作岗位',
  `MGR` int(4) DEFAULT NULL COMMENT '上级经理',
  `HIREDATE` date DEFAULT NULL,
  `SAL` double(7,2) DEFAULT NULL,
  `COMM` double(7,2) DEFAULT NULL,
  `DEPTNO` int(2) DEFAULT NULL,
  PRIMARY KEY (`EMPNO`),
  KEY `DEPTNO` (`DEPTNO`),
  KEY `SAL` (`SAL`),
  CONSTRAINT `emp_ibfk_1` FOREIGN KEY (`DEPTNO`) REFERENCES `dept` (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='员工编号';

插入语句：

INSERT INTO `emp` VALUES ('7369', 'SMITH', 'CLERK', '7902', '1980-12-17', '800.00', null, '20');
INSERT INTO `emp` VALUES ('7499', 'ALLEN', 'SALESMAN', '7698', '1981-02-20', '1600.00', '300.00', '30');
INSERT INTO `emp` VALUES ('7521', 'WARD', 'SALESMAN', '7698', '1981-02-22', '1250.00', '500.00', '30');
INSERT INTO `emp` VALUES ('7566', 'JONES', 'MANAGER', '7839', '1981-04-02', '2975.00', null, '20');
INSERT INTO `emp` VALUES ('7654', 'MARTIN', 'SALESMAN', '7698', '1981-09-28', '1250.00', '1400.00', '30');
INSERT INTO `emp` VALUES ('7698', 'BLAKE', 'MANAGER', '7839', '1981-05-01', '2850.00', null, '30');
INSERT INTO `emp` VALUES ('7782', 'CLARK', 'MANAGER', '7839', '1981-06-09', '2450.00', null, '10');
INSERT INTO `emp` VALUES ('7788', 'SCOTT', 'ANALYST', '7566', '1987-04-19', '3000.00', null, '20');
INSERT INTO `emp` VALUES ('7839', 'KING', 'PRESIDENT', null, '1981-11-17', '5000.00', null, '10');
INSERT INTO `emp` VALUES ('7844', 'TURNER', 'SALESMAN', '7698', '1981-09-08', '1500.00', '0.00', '30');
INSERT INTO `emp` VALUES ('7876', 'ADAMS', 'CLERK', '7788', '1981-05-23', '1100.00', null, '20');
INSERT INTO `emp` VALUES ('7900', 'JAMES', 'CLERK', '7698', '1981-12-03', '950.00', null, '30');
INSERT INTO `emp` VALUES ('7902', 'FORD', 'ANALYST', '7566', '1981-12-03', '3000.00', null, '20');
INSERT INTO `emp` VALUES ('7934', 'MILLER', 'CLERK', '7782', '1982-01-23', '1300.00', null, '10');

C:薪水等级表

1，建表语句

DROP TABLE IF EXISTS `salgrade`;
CREATE TABLE `salgrade` (
  `GRADE` int(11) DEFAULT NULL,
  `LOSAL` int(11) DEFAULT NULL,
  `HISAL` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `salgrade` VALUES ('1', '700', '1200');
INSERT INTO `salgrade` VALUES ('2', '1201', '1400');
INSERT INTO `salgrade` VALUES ('3', '1401', '2000');
INSERT INTO `salgrade` VALUES ('4', '2001', '3000');
INSERT INTO `salgrade` VALUES ('5', '3001', '9999');

表的结构如下所示：

1.取得每个部门最高薪水的人员名称

第一步：求出每个部门的最高薪水
select
    e.deptno,max(e.sal) as maxsal
from
    emp e
group by
    e.deptno;
+--------+---------+
| deptno | maxsal  |
+--------+---------+
|     10 | 5000.00 |
|     20 | 3000.00 |
|     30 | 2850.00 |
+--------+---------+
将以上查询结果当成一个临时表t(deptno,maxsal)
select 
    e.deptno,e.ename,t.maxsal,e.sal
from
    (select
            e.deptno,max(e.sal) as maxsal
        from
            emp e
        group by
            e.deptno)t
join
    emp e
on
    t.deptno = e.deptno
where
    t.maxsal = e.sal
order by
    e.deptno;
+--------+-------+---------+---------+
| deptno | ename | maxsal  | sal     |
+--------+-------+---------+---------+
|     10 | KING  | 5000.00 | 5000.00 |
|     20 | SCOTT | 3000.00 | 3000.00 |
|     20 | FORD  | 3000.00 | 3000.00 |
|     30 | BLAKE | 2850.00 | 2850.00 |
+--------+-------+---------+---------+

分析下：

首先group by 首先经常和聚合函数max等配合使用，第二使用了group by 在select后面的查询字段只能是group by 后面指定的字段不能是其他字段

第三：join on 条件中 on 和where的却别，不清楚的看自己的博客

MYSQL LEFT JOIN操作中 ON与WHERE放置条件的区别

on是两个表联合查询连接起来生成一个临时表，where是在生成临时表的基础上，对生成的临时表进行条件帅选

  t.deptno = e.deptno 表示两个表生成临时表的关系是 emp表中的部门编号必须等于 t表中的部门编号
where之后的条件是：emp表和t表已经生成了临时表，然后对临时表进行条件过滤

2.哪些人的薪水在部门平均薪水之上

2.哪些人的薪水在部门平均薪水之上
第一步：求出每个部门的平均薪水
select
    e.deptno,avg(e.sal) as avgsal
from
    emp e
group by 
    e.deptno;
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
将以上查询结果当成临时表t(deptno,avgsal)

select
    t.deptno,e.ename
from
    (select
        e.deptno,avg(e.sal) as avgsal
    from
        emp e
    group by 
        e.deptno) t
join
    emp e
on
    e.deptno = t.deptno
where
    e.sal > t.avgsal;
+--------+-------+
| deptno | ename |
+--------+-------+
|     30 | ALLEN |
|     20 | JONES |
|     30 | BLAKE |
|     20 | SCOTT |
|     10 | KING  |
|     20 | FORD  |
+--------+-------+

3.取得部门中（所有人的）平均薪水等级

第一种情况：emp表中按照部门进行分组，求出每个组的平均工资，看每个组的平均工资属于那个等级

第一步：求出部门的平均薪水
select
    e.deptno,avg(e.sal) as avgsal
from
    emp e
group by
    e.deptno;
将以下查询结果当成临时表t(deptno,avgsal)
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+

select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+

select 
    t.deptno,t.avgsal,s.grade
from
    (select
            e.deptno,avg(e.sal) as avgsal
        from
            emp e
        group by
            e.deptno) t
join
    salgrade s
on
    t.avgsal between s.losal and s.hisal;
+--------+-------------+-------+
| deptno | avgsal      | grade |
+--------+-------------+-------+
|     30 | 1566.666667 |     3 |
|     10 | 2916.666667 |     4 |
|     20 | 2175.000000 |     4 |
+--------+-------------+-------+

第二种情况：首先求出每个人的薪水属于那个等级，然后进行分组

3.2 取得部门中所有人的平均的薪水等级
第一步：求出每个人的薪水等级
select 
    e.deptno,e.ename,s.grade
from
    emp e
join
    salgrade s
on
    e.sal between s.losal and s.hisal
order by
    e.deptno;
+--------+--------+-------+
| deptno | ename  | grade |
+--------+--------+-------+
|     10 | CLARK  |     4 |
|     10 | MILLER |     2 |
|     10 | KING   |     5 |
|     20 | ADAMS  |     1 |
|     20 | SMITH  |     1 |
|     20 | FORD   |     4 |
|     20 | SCOTT  |     4 |
|     20 | JONES  |     4 |
|     30 | BLAKE  |     4 |
|     30 | JAMES  |     1 |
|     30 | ALLEN  |     3 |
|     30 | WARD   |     2 |
|     30 | TURNER |     3 |
|     30 | MARTIN |     2 |
+--------+--------+-------+
将以上查询结果当成临时表t(deptno,ename,grade)
select 
    t.deptno,avg(t.grade) as avgGrade
from
    (select 
        e.deptno,e.ename,s.grade
    from
        emp e
    join
        salgrade s
    on
        e.sal between s.losal and s.hisal) t
group by
    t.deptno;
+--------+----------+
| deptno | avgGrade |
+--------+----------+
|     10 |   3.6667 |
|     20 |   2.8000 |
|     30 |   2.5000 |
+--------+----------+

4.不准用组函数（MAX），取得最高薪水（给出两种解决方案）
select sal from emp order by sal desc limit 1;

5.取得平均薪水最高的部门的部门编号

5.取得平均薪水最高的部门的部门编号
第一步：求出部门平均薪水
select
    e.deptno,avg(e.sal) as avgsal
from
    emp e
group by
    e.deptno;
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
第二步：将以上查询结果当成临时表t(deptno,avgsal),求出最高的平均薪水
select max(t.avgsal) as maxAvgSal from (select
                        e.deptno,avg(e.sal) as avgsal
                    from
                        emp e
                    group by
                        e.deptno) t;
+-------------+
| maxAvgSal   |
+-------------+
| 2916.666667 |
+-------------+

select
    e.deptno,avg(e.sal) as avgsal
from
    emp e
group by
    e.deptno
having 
    avgsal = (select max(t.avgsal) as maxAvgSal from (select
                        e.deptno,avg(e.sal) as avgsal
                    from
                        emp e
                    group by
                        e.deptno) t);
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
+--------+-------------+

主要这里有一个坑不能写成下面的形式：

select 
 e.deptno,avg(e.sal) avgSal  
from 
     emp e
group by
     e.deptno
order by
    avgSal desc
limit 1;

因为如果有100个部门，可能存在很多个部门的平均值都是一样的

6.取得平均薪水最高的部门的部门名称
select
    e.deptno,d.dname,avg(e.sal) as avgsal
from
    emp e
join
    dept d
on
    e.deptno = d.deptno
group by
    e.deptno,d.dname
having 
    avgsal = (select max(t.avgsal) as maxAvgSal from (select
                        e.deptno,avg(e.sal) as avgsal
                    from
                        emp e
                    group by
                        e.deptno) t);

7.求平均薪水的等级最低的部门的部门名称
第一步：部门的平均薪水
select 
    e.deptno,d.dname,avg(e.sal) as avgsal
from
    emp e
join
    dept d
on
    e.deptno = d.deptno
group by
    e.deptno,d.dname;
+--------+------------+-------------+
| deptno | dname      | avgsal      |
+--------+------------+-------------+
|     10 | ACCOUNTING | 2916.666667 |
|     20 | RESEARCH   | 2175.000000 |
|     30 | SALES      | 1566.666667 |
+--------+------------+-------------+
第二步：将以上结果当成临时表t(deptno,avgsal)与salgrade表进行表连接：t.avgsal between s.losal and s.hisal;
select 
    t.deptno,t.dname,s.grade
from    
    (select 
            e.deptno,d.dname,avg(e.sal) as avgsal
        from
            emp e
        join
            dept d
        on
            e.deptno = d.deptno
        group by
            e.deptno,d.dname)t
join
    salgrade s
on
    t.avgsal between s.losal and s.hisal;
+--------+------------+-------+
| deptno | dname      | grade |
+--------+------------+-------+
|     30 | SALES      |     3 |
|     10 | ACCOUNTING |     4 |
|     20 | RESEARCH   |     4 |
+--------+------------+-------+

第三步：将以上查询结果当成一张临时表t
select min(t.grade) as minGrade from (select 
                    t.deptno,t.dname,s.grade
                from    
                    (select 
                            e.deptno,d.dname,avg(e.sal) as avgsal
                        from
                            emp e
                        join
                            dept d
                        on
                            e.deptno = d.deptno
                        group by
                            e.deptno,d.dname)t
                join
                    salgrade s
                on
                    t.avgsal between s.losal and s.hisal)t;
+----------+
| minGrade |
+----------+
|        3 |
+----------+

select 
    t.deptno,t.dname,s.grade
from    
    (select 
            e.deptno,d.dname,avg(e.sal) as avgsal
        from
            emp e
        join
            dept d
        on
            e.deptno = d.deptno
        group by
            e.deptno,d.dname)t
join
    salgrade s
on
    t.avgsal between s.losal and s.hisal
where
    s.grade = (select min(t.grade) as minGrade from (select 
                    t.deptno,t.dname,s.grade
                from    
                    (select 
                            e.deptno,d.dname,avg(e.sal) as avgsal
                        from
                            emp e
                        join
                            dept d
                        on
                            e.deptno = d.deptno
                        group by
                            e.deptno,d.dname)t
                join
                    salgrade s
                on
                    t.avgsal between s.losal and s.hisal)t);
+--------+-------+-------+
| deptno | dname | grade |
+--------+-------+-------+
|     30 | SALES |     3 |
+--------+-------+-------+

8.取得比普通员工（员工代码没有在mgr上出现的）的最高薪水还要高的经理人姓名
第一步：找出普通员工(员工代码没有出现在mgr上的)
    1.1 先找出mgr有哪些人
    select distinct mgr from emp;
        +------+
        | mgr  |
        +------+
        | 7902 |
        | 7698 |
        | 7839 |
        | 7566 |
        | NULL |
        | 7788 |
        | 7782 |
        +------+
select * from emp where empno in(select distinct mgr from emp);
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
|  7782 | CLARK | MANAGER   | 7839 | 1981-06-09 | 2450.00 | NULL |     10 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+------------+---------+------+--------+
select max(sal) as maxsal from emp where empno not in(select distinct mgr from emp where mgr is not null);
+---------+
| maxsal  |
+---------+
| 1600.00 |
+---------+

not in不会自动忽略空值
in会自动忽略空值


select ename from emp where sal > (select max(sal) as maxsal from emp where empno not in(select distinct mgr from emp where mgr is not null));
+-------+
| ename |
+-------+
| JONES |
| BLAKE |
| CLARK |
| SCOTT |
| KING  |
| FORD  |
+-------+

第一步：找出普通员工(员工代码没有出现在mgr上的)
    1.1 先找出mgr有哪些人
    select distinct mgr from emp;
        +------+
        | mgr  |
        +------+
        | 7902 |
        | 7698 |
        | 7839 |
        | 7566 |
        | NULL |
        | 7788 |
        | 7782 |
        +------+
select * from emp where empno in(select distinct mgr from emp);
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
|  7782 | CLARK | MANAGER   | 7839 | 1981-06-09 | 2450.00 | NULL |     10 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+------------+---------+------+--------+
select max(sal) as maxsal from emp where empno not in(select distinct mgr from emp where mgr is not null);
+---------+
| maxsal  |
+---------+
| 1600.00 |
+---------+

not in不会自动忽略空值
in会自动忽略空值


select ename from emp where sal > (select max(sal) as maxsal from emp where empno not in(select distinct mgr from emp where mgr is not null));
+-------+
| ename |
+-------+
| JONES |
| BLAKE |
| CLARK |
| SCOTT |
| KING  |
| FORD  |
+-------+

这里有一个很关键的地方，not in 没有排除null值，如果存在null值和not in 做计算，得带的值就是null

mysql> select * from emp where empno not in(select distinct mgr from emp);
Empty set

mysql> 

select distinct mgr from emp 的结果存在null值

这里not in  没有去掉null值得到的结果就是空

9.取得薪水最高的前五名员工
select * from emp order by sal desc limit 0,5;
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
+-------+-------+-----------+------+------------+---------+------+--------+

10.取得薪水最高的第六到第十名员工

select * from emp order by sal desc limit 5,5;
+-------+--------+----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB      | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+----------+------+------------+---------+---------+--------+
|  7782 | CLARK  | MANAGER  | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|  7499 | ALLEN  | SALESMAN | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7934 | MILLER | CLERK    | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
|  7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
+-------+--------+----------+------+------------+---------+---------+--------+

11.取得最后入职的5名员工
select * from emp order by hiredate desc limit 5;
+-------+--------+---------+------+------------+---------+------+--------+
| EMPNO | ENAME  | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+--------+---------+------+------------+---------+------+--------+
|  7876 | ADAMS  | CLERK   | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
|  7788 | SCOTT  | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7934 | MILLER | CLERK   | 7782 | 1982-01-23 | 1300.00 | NULL |     10 |
|  7902 | FORD   | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
|  7900 | JAMES  | CLERK   | 7698 | 1981-12-03 |  950.00 | NULL |     30 |
+-------+--------+---------+------+------------+---------+------+--------+

12.取得每个薪水等级有多少员工
第一步：查询出每个员工的薪水等级
select 
    e.ename,s.grade 
from 
    emp e
join
    salgrade s
on
    e.sal between s.losal and s.hisal
order by
    s.grade;
+--------+-------+
| ename  | grade |
+--------+-------+
| JAMES  |     1 |
| SMITH  |     1 |
| ADAMS  |     1 |
| MILLER |     2 |
| WARD   |     2 |
| MARTIN |     2 |
| ALLEN  |     3 |
| TURNER |     3 |
| BLAKE  |     4 |
| FORD   |     4 |
| CLARK  |     4 |
| SCOTT  |     4 |
| JONES  |     4 |
| KING   |     5 |
+--------+-------+

将以上查询结果当成临时表t(ename,grade)
select
    t.grade,count(t.ename) as totalEmp
from
    (select 
        e.ename,s.grade 
    from 
        emp e
    join
        salgrade s
    on
        e.sal between s.losal and s.hisal) t
group by
    t.grade;
+-------+----------+
| grade | totalEmp |
+-------+----------+
|     1 |        3 |
|     2 |        3 |
|     3 |        2 |
|     4 |        5 |
|     5 |        1 |
+-------+----------+