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 ('', 'ACCOUNTING', 'NEW YORK');

INSERT INTO `dept` VALUES ('', 'RESEARCH', 'DALLAS');

INSERT INTO `dept` VALUES ('', 'SALES', 'CHICAGO');

INSERT INTO `dept` VALUES ('', '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 ('', 'SMITH', 'CLERK', '', '1980-12-17', '800.00', null, '');

INSERT INTO `emp` VALUES ('', 'ALLEN', 'SALESMAN', '', '1981-02-20', '1600.00', '300.00', '');

INSERT INTO `emp` VALUES ('', 'WARD', 'SALESMAN', '', '1981-02-22', '1250.00', '500.00', '');

INSERT INTO `emp` VALUES ('', 'JONES', 'MANAGER', '', '1981-04-02', '2975.00', null, '');

INSERT INTO `emp` VALUES ('', 'MARTIN', 'SALESMAN', '', '1981-09-28', '1250.00', '1400.00', '');

INSERT INTO `emp` VALUES ('', 'BLAKE', 'MANAGER', '', '1981-05-01', '2850.00', null, '');

INSERT INTO `emp` VALUES ('', 'CLARK', 'MANAGER', '', '1981-06-09', '2450.00', null, '');

INSERT INTO `emp` VALUES ('', 'SCOTT', 'ANALYST', '', '1987-04-19', '3000.00', null, '');

INSERT INTO `emp` VALUES ('', 'KING', 'PRESIDENT', null, '1981-11-17', '5000.00', null, '');

INSERT INTO `emp` VALUES ('', 'TURNER', 'SALESMAN', '', '1981-09-08', '1500.00', '0.00', '');

INSERT INTO `emp` VALUES ('', 'ADAMS', 'CLERK', '', '1981-05-23', '1100.00', null, '');

INSERT INTO `emp` VALUES ('', 'JAMES', 'CLERK', '', '1981-12-03', '950.00', null, '');

INSERT INTO `emp` VALUES ('', 'FORD', 'ANALYST', '', '1981-12-03', '3000.00', null, '');

INSERT INTO `emp` VALUES ('', 'MILLER', 'CLERK', '', '1982-01-23', '1300.00', null, '');

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 ('', '', '');

INSERT INTO `salgrade` VALUES ('', '', '');

INSERT INTO `salgrade` VALUES ('', '', '');

INSERT INTO `salgrade` VALUES ('', '', '');

INSERT INTO `salgrade` VALUES ('', '', '');

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 |

+-------+----------+

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