MySQL 多表查询与复杂查询技巧实战

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发布于 2025-03-24 11:06:44

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练习相关的数据

DROP database IF EXISTS `scott`;
CREATE database IF NOT EXISTS `scott` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;

USE `scott`;

DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
  `deptno` int(2) unsigned zerofill NOT NULL COMMENT '部门编号',
  `dname` varchar(14) DEFAULT NULL COMMENT '部门名称',
  `loc` varchar(13) DEFAULT NULL COMMENT '部门所在地点'
);


DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (
  `empno` int(6) unsigned zerofill NOT NULL COMMENT '雇员编号',
  `ename` varchar(10) DEFAULT NULL COMMENT '雇员姓名',
  `job` varchar(9) DEFAULT NULL COMMENT '雇员职位',
  `mgr` int(4) unsigned zerofill DEFAULT NULL COMMENT '雇员领导编号',
  `hiredate` datetime DEFAULT NULL COMMENT '雇佣时间',
  `sal` decimal(7,2) DEFAULT NULL COMMENT '工资月薪',
  `comm` decimal(7,2) DEFAULT NULL COMMENT '奖金',
  `deptno` int(2) unsigned zerofill DEFAULT NULL COMMENT '部门编号'
);


DROP TABLE IF EXISTS `salgrade`;
CREATE TABLE `salgrade` (
  `grade` int(11) DEFAULT NULL COMMENT '等级',
  `losal` int(11) DEFAULT NULL COMMENT '此等级最低工资',
  `hisal` int(11) DEFAULT NULL COMMENT '此等级最高工资'
);


insert into dept (deptno, dname, loc)
values (10, 'ACCOUNTING', 'NEW YORK');
insert into dept (deptno, dname, loc)
values (20, 'RESEARCH', 'DALLAS');
insert into dept (deptno, dname, loc)
values (30, 'SALES', 'CHICAGO');
insert into dept (deptno, dname, loc)
values (40, 'OPERATIONS', 'BOSTON');

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, null, 20);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7844, 'TURNER', 'SALESMAN', 7698,'1981-09-08', 1500, 0, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, null, 20);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);

insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);

insert into salgrade (grade, losal, hisal) values (1, 700, 1200);
insert into salgrade (grade, losal, hisal) values (2, 1201, 1400);
insert into salgrade (grade, losal, hisal) values (3, 1401, 2000);
insert into salgrade (grade, losal, hisal) values (4, 2001, 3000);
insert into salgrade (grade, losal, hisal) values (5, 3001, 9999);

基本查询回顾

查询工资高于500或岗位为MANAGER的雇员，同时还要满足他们的姓名首字母为大写的J

select * from emp where (sal > 500 or job='MANAGER') and ename like 'J%';
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
+--------+-------+---------+------+---------------------+---------+------+--------+
2 rows in set (0.00 sec)

按照部门号升序而雇员的工资降序排序

select * from emp order by deptno, sal desc;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)

使用年薪进行降序排序年薪 = cal + comm

select ename, sal*12+ifnull(comm,0) as '年薪' from emp order by 年薪 desc;
+--------+----------+
| ename  | 年薪     |
+--------+----------+
| KING   | 60000.00 |
| SCOTT  | 36000.00 |
| FORD   | 36000.00 |
| JONES  | 35700.00 |
| BLAKE  | 34200.00 |
| CLARK  | 29400.00 |
| ALLEN  | 19500.00 |
| TURNER | 18000.00 |
| MARTIN | 16400.00 |
| MILLER | 15600.00 |
| WARD   | 15500.00 |
| ADAMS  | 13200.00 |
| JAMES  | 11400.00 |
| SMITH  |  9600.00 |
+--------+----------+
14 rows in set (0.00 sec)

显示工资最高的员工的名字和工作岗位

select ename, job from emp where sal = (select max(sal) from emp);
+-------+-----------+
| ename | job       |
+-------+-----------+
| KING  | PRESIDENT |
+-------+-----------+
1 row in set (0.00 sec)

显示工资高于平均工资的员工信息

select ename, sal from emp where sal > (select avg(sal) from emp);
+-------+---------+
| ename | sal     |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING  | 5000.00 |
| FORD  | 3000.00 |
+-------+---------+
6 rows in set (0.00 sec)

显示每个部门的平均工资和最高工资

select deptno, format(avg(sal), 2) , max(sal) from emp group by deptno;
+--------+---------------------+----------+
| deptno | format(avg(sal), 2) | max(sal) |
+--------+---------------------+----------+
|     10 | 2,916.67            |  5000.00 |
|     20 | 2,175.00            |  3000.00 |
|     30 | 1,566.67            |  2850.00 |
+--------+---------------------+----------+
3 rows in set (0.00 sec)

显示平均工资低于2000的部门号和它的平均工资

select deptno, avg(sal) as '平均工资' from emp group by deptno having 平均工资 < 2000;
+--------+--------------+
| deptno | 平均工资     |
+--------+--------------+
|     30 |  1566.666667 |
+--------+--------------+
1 row in set (0.00 sec)

显示每种岗位的雇员总数，平均工资

select job, count(*) ,format(avg(sal), 2) from emp group by job;
+-----------+----------+---------------------+
| job       | count(*) | format(avg(sal), 2) |
+-----------+----------+---------------------+
| ANALYST   |        2 | 3,000.00            |
| CLERK     |        4 | 1,037.50            |
| MANAGER   |        3 | 2,758.33            |
| PRESIDENT |        1 | 5,000.00            |
| SALESMAN  |        4 | 1,400.00            |
+-----------+----------+---------------------+
5 rows in set (0.00 sec)

多表查询

实际开发中往往数据来自不同的表，所以需要多表查询。本节我们用一个简单的公司管理系统，有三张表EMP,DEPT,SALGRADE来演示如何进行多表查询。

案例：显示雇员名、雇员工资以及所在部门的名字因为上面的数据来自EMP和DEPT表，因此要联合查询

其实我们只要emp表中的deptno = dept表中的deptno字段的记录

显示部门号为10的部门名，员工名和工资

mysql> select ename, sal,dname from emp, dept where emp.deptno=dept.deptno and
    -> dept.deptno = 10;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| CLARK  | 2450.00 | ACCOUNTING |
| KING   | 5000.00 | ACCOUNTING |
| MILLER | 1300.00 | ACCOUNTING |
+--------+---------+------------+
3 rows in set (0.00 sec)

显示各个员工的姓名，工资，及工资级别

mysql> select ename, sal, grade from emp, salgrade where emp.sal between losal and
    -> hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.00 sec)

自连接

自连接是指在同一张表连接查询案例：显示员工FORD的上级领导的编号和姓名（mgr是员工领导的编号–empno）

使用的子查询：

mysql> select empno,ename from emp where emp.empno=(select mgr from emp where 
    -> ename='FORD');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

使用多表查询（自查询）

mysql> select leader.empno,leader.ename from emp leader, emp worker where 
    -> leader.empno = worker.mgr and worker.ename='FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

子查询

子查询是指嵌入在其他sql语句中的select语句，也叫嵌套查询

单行子查询

返回一行记录的子查询

显示SMITH同一部门的员工

多行子查询

返回多行记录的子查询

in关键字；查询和10号部门的工作岗位相同的雇员的名字，岗位，工资，部门号，但是不包含10自己的

all关键字；显示工资比部门30的所有员工的工资高的员工的姓名、工资和部门号

any关键字；显示工资比部门30的任意员工的工资高的员工的姓名、工资和部门号（包含自己部门的员工）

多列子查询

单行子查询是指子查询只返回单列，单行数据；多行子查询是指返回单列多行数据，都是针对单列而言的，而多列子查询则是指查询返回多个列数据的子查询语句案例：查询和SMITH的部门和岗位完全相同的所有雇员，不含SMITH本人

mysql> select ename from emp where (deptno, job)=(select deptno, job from emp
    -> where ename='SMITH') and ename <> 'SMITH';
+-------+
| ename |
+-------+
| ADAMS |
+-------+
1 row in set (0.00 sec)

在from子句中使用子查询

子查询语句出现在from子句中。这里要用到数据查询的技巧，把一个子查询当做一个临时表使用。

案例：

显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资

mysql> select ename, deptno, sal, format(asal,2) from emp,
    ->  (select avg(sal) asal, deptno dt from emp group by deptno) tmp
    -> where emp.sal > tmp.asal and emp.deptno=tmp.dt
    -> ;
+-------+--------+---------+----------------+
| ename | deptno | sal     | format(asal,2) |
+-------+--------+---------+----------------+
| KING  |     10 | 5000.00 | 2,916.67       |
| JONES |     20 | 2975.00 | 2,175.00       |
| SCOTT |     20 | 3000.00 | 2,175.00       |
| FORD  |     20 | 3000.00 | 2,175.00       |
| ALLEN |     30 | 1600.00 | 1,566.67       |
| BLAKE |     30 | 2850.00 | 1,566.67       |
+-------+--------+---------+----------------+
6 rows in set (0.00 sec)

查找每个部门工资最高的人的姓名、工资、部门、最高工资

mysql> select emp.ename, emp.sal, emp.deptno, ms from emp,
    -> (select max(sal) ms, deptno from emp group by deptno) tmp
    ->  where emp.deptno=tmp.deptno and emp.sal=tmp.ms;
+-------+---------+--------+---------+
| ename | sal     | deptno | ms      |
+-------+---------+--------+---------+
| BLAKE | 2850.00 |     30 | 2850.00 |
| SCOTT | 3000.00 |     20 | 3000.00 |
| KING  | 5000.00 |     10 | 5000.00 |
| FORD  | 3000.00 |     20 | 3000.00 |
+-------+---------+--------+---------+
4 rows in set (0.00 sec)

显示每个部门的信息（部门名，编号，地址）和人员数量

方法1：使用多表

select DEPT.dname, DEPT.deptno, DEPT.loc,count(*) '部门人数' from EMP, 
DEPT
where EMP.deptno=DEPT.deptno
group by DEPT.deptno,DEPT.dname,DEPT.loc;

方法2：使用子查询

-- 1. 对EMP表进行人员统计
select count(*), deptno from EMP group by deptno;
-- 2. 将上面的表看作临时表
select DEPT.deptno, dname, mycnt, loc from DEPT, 
(select count(*) mycnt, deptno from EMP group by deptno) tmp
where DEPT.deptno=tmp.deptno;

合并查询

在实际应用中，为了合并多个select的执行结果，可以使用集合操作符 union，union all

union 该操作符用于取得两个结果集的并集。当使用该操作符时，会自动去掉结果集中的重复行。

案例：将工资大于2500或职位是MANAGER的人找出来

mysql> select ename, sal, job from EMP where sal>2500 union 
 -> select ename, sal, job from EMP where job='MANAGER';--去掉了重复记录
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+

union all 该操作符用于取得两个结果集的并集。当使用该操作符时，不会去掉结果集中的重复行。

案例：将工资大于25000或职位是MANAGER的人找出来

mysql> select ename, sal, job from EMP where sal>2500 union all 
 -> select ename, sal, job from EMP where job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING | 5000.00 | PRESIDENT |
| FORD | 3000.00 | ANALYST   |
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+

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原始发表：2025-03-23，如有侵权请联系 cloudcommunity@tencent.com 删除

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