【MySQL】复合查询

JhonKI2024-12-12 8:56

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文章目录

  • 📢前言
  • [🏳️‍🌈1 基本查询回顾](#🏳️‍🌈1 基本查询回顾)
  • [🏳️‍🌈2 多表查询](#🏳️‍🌈2 多表查询)
  • [🏳️‍🌈3 自连接](#🏳️‍🌈3 自连接)
  • [🏳️‍🌈4 子查询](#🏳️‍🌈4 子查询)
    • [❤️4.1 单行子查询](#❤️4.1 单行子查询)
    • [🧡4.2 多行子查询](#🧡4.2 多行子查询)
    • [💛4.3 多列子查询](#💛4.3 多列子查询)
    • [💚4.4 在from子句中使用子查询](#💚4.4 在from子句中使用子查询)
    • [💙4.5 合并查询](#💙4.5 合并查询)
      • [4.5.1 union](#4.5.1 union)
      • [4.5.3 union all](#4.5.3 union all)
  • 👥总结

📢前言

MySQL 复合函数是一组强大的数据处理工具，可以对数据库中的数据进行统计、计算和分组等操作。这些函数包括聚合函数、流程控制语句和复合查询等，可以帮助开发人员更高效地处理数据。

MySQL 复合函数与 GROUP BY 的关系在使用 MySQL 进行数据处理和查询时，复合函数（如聚合函数）是一种常见的功能。复合函数可用于汇总数据，比如计算总和、平均数、最大值、最小值等。那么，在使用这些函数时，是否必须使用 GROUP BY 语句呢？本文将对此进行深入探讨，并提供代码示例以帮助理解。

本文用到的数据表

cpp 复制代码 
// emp表
mysql> show create table emp;
+-------+--------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
+-------+--------------------------------------------------------------------------------------------------+
| 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'
) ENGINE=InnoDB DEFAULT CHARSET=utf8                                 |
+-------+--------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

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



// // dept
mysql> show create table dept;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                             |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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'
) ENGINE=InnoDB DEFAULT CHARSET=utf8               |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select * from dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.00 sec)

// salgrade
mysql> show create table salgrade;
+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                                                                   |
+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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'
) ENGINE=InnoDB DEFAULT CHARSET=utf8                 |
+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select * from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)

🏳️‍🌈1 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

cpp 复制代码 
mysql> 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)

mysql> select * from emp where(sal > 500 or job = 'MANAGER') and substring(ename, 1, 1)='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

cpp 复制代码 
mysql> 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

cpp 复制代码 
mysql> 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

cpp 复制代码 
// 子查询
mysql> select ename, job, sal from emp where sal = (select(max(sal)) from emp);
+-------+-----------+---------+
| ename | job       | sal     |
+-------+-----------+---------+
| KING  | PRESIDENT | 5000.00 |
+-------+-----------+---------+
1 row in set (0.00 sec)

显示工资高于平均工资的员工信息

cpp 复制代码 
// 子查询
mysql> select * from emp where sal > (select avg(sal) from emp);
+--------+-------+-----------+------+---------------------+---------+------+--------+
| empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-----------+------+---------------------+---------+------+--------+
| 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
| 007782 | CLARK | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
| 007788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
| 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+-----------+------+---------------------+---------+------+--------+
6 rows in set (0.00 sec)

显示每个部门的平均工资和最高工资

cpp 复制代码 
mysql> 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

cpp 复制代码 
mysql> select deptno, avg(sal) 平均 from emp group by deptno;
+--------+-------------+
| deptno | 平均        |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.01 sec)

mysql> select deptno, avg(sal) 平均 from emp group by deptno having avg(sal) < 2000;
+--------+-------------+
| deptno | 平均        |
+--------+-------------+
|     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

cpp 复制代码 
mysql> select job, count(*), avg(sal) 平均工资 from emp group by job;
+-----------+----------+--------------+
| job       | count(*) | 平均工资     |
+-----------+----------+--------------+
| ANALYST   |        2 |  3000.000000 |
| CLERK     |        4 |  1037.500000 |
| MANAGER   |        3 |  2758.333333 |
| PRESIDENT |        1 |  5000.000000 |
| SALESMAN  |        4 |  1400.000000 |
+-----------+----------+--------------+
5 rows in set (0.00 sec)

🏳️‍🌈2 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。

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

cpp 复制代码 
mysql> select emp.ename, emp.sal, emp.deptno, dept.dname from emp, dept where emp.deptno = dept.deptno;

+--------+---------+--------+------------+
| ename  | sal     | deptno | dname      |
+--------+---------+--------+------------+
| SMITH  |  800.00 |     20 | RESEARCH   |
| ALLEN  | 1600.00 |     30 | SALES      |
| WARD   | 1250.00 |     30 | SALES      |
| JONES  | 2975.00 |     20 | RESEARCH   |
| MARTIN | 1250.00 |     30 | SALES      |
| BLAKE  | 2850.00 |     30 | SALES      |
| CLARK  | 2450.00 |     10 | ACCOUNTING |
| SCOTT  | 3000.00 |     20 | RESEARCH   |
| KING   | 5000.00 |     10 | ACCOUNTING |
| TURNER | 1500.00 |     30 | SALES      |
| ADAMS  | 1100.00 |     20 | RESEARCH   |
| JAMES  |  950.00 |     30 | SALES      |
| FORD   | 3000.00 |     20 | RESEARCH   |
| MILLER | 1300.00 |     10 | ACCOUNTING |
+--------+---------+--------+------------+
14 rows in set (0.00 sec)

// 再根据 deptno 排序
mysql> select emp.ename, emp.sal, emp.deptno, dept.dname from emp, dept where emp.deptno = dept.deptno order by deptno;
+--------+---------+--------+------------+
| ename  | sal     | deptno | dname      |
+--------+---------+--------+------------+
| CLARK  | 2450.00 |     10 | ACCOUNTING |
| MILLER | 1300.00 |     10 | ACCOUNTING |
| KING   | 5000.00 |     10 | ACCOUNTING |
| SMITH  |  800.00 |     20 | RESEARCH   |
| JONES  | 2975.00 |     20 | RESEARCH   |
| FORD   | 3000.00 |     20 | RESEARCH   |
| SCOTT  | 3000.00 |     20 | RESEARCH   |
| ADAMS  | 1100.00 |     20 | RESEARCH   |
| TURNER | 1500.00 |     30 | SALES      |
| ALLEN  | 1600.00 |     30 | SALES      |
| MARTIN | 1250.00 |     30 | SALES      |
| WARD   | 1250.00 |     30 | SALES      |
| BLAKE  | 2850.00 |     30 | SALES      |
| JAMES  |  950.00 |     30 | SALES      |
+--------+---------+--------+------------+
14 rows in set (0.00 sec)

显示部门号为10的部门名，员工名和工资

  • 在第一个查询select dname, ename, sal from emp, dept where emp.deptno = dept.deptno and emp.deptno = 10;中，列名dname、ename和sal没有指定表前缀。在这种情况下，MySQL 会按照from子句中表出现的顺序来确定列所属的表。这里它会先在emp表中查找列，找不到就会在dept表中查找。
  • 第二个查询select dept.dname, emp.ename, emp.sal from emp, dept where emp.deptno = dept.deptno and emp.deptno = 10;明确使用了表前缀（dept.dname、emp.ename和emp.sal）来指定列所属的表。这种方式更加清晰，尤其是在涉及多个表且列名可能重复的情况下，可以避免歧义。
cpp 复制代码 
mysql> select dname, ename, sal from emp, dept where emp.deptno=dept.deptno and emp.deptno = 10;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| ACCOUNTING | CLARK  | 2450.00 |
| ACCOUNTING | KING   | 5000.00 |
| ACCOUNTING | MILLER | 1300.00 |
+------------+--------+---------+
3 rows in set (0.00 sec)

mysql> select dept.dname, emp.ename, emp.sal from emp, dept where emp.deptno=dept.deptno and emp.deptno
 = 10;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| ACCOUNTING | CLARK  | 2450.00 |
| ACCOUNTING | KING   | 5000.00 |
| ACCOUNTING | MILLER | 1300.00 |
+------------+--------+---------+
3 rows in set (0.01 sec)

显示各个员工的姓名，工资，及工资级别

cpp 复制代码 
mysql> select * from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)

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)

🏳️‍🌈3 自连接

自连接是指在同一张表连接查询

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
cpp 复制代码 
mysql> select empno, ename from emp where 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）
cpp 复制代码 
// 使用到表的别名
// from emp leader, emp worker，给自己的表起别名，因为要先做笛卡尔积，所以别名可以先识别
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)

🏳️‍🌈4 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

❤️4.1 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

cpp 复制代码 
mysql> select * from emp where deptno = (select deptno from emp where ename = 'SMITH');
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007876 | ADAMS | CLERK   | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
| 007902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.00 sec)

🧡4.2 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
cpp 复制代码 
mysql> select ename, job, sal, deptno from emp  where job in (select distinct job from emp where deptno
 = 10) and deptno<>10;
+-------+---------+---------+--------+
| ename | job     | sal     | deptno |
+-------+---------+---------+--------+
| JONES | MANAGER | 2975.00 |     20 |
| BLAKE | MANAGER | 2850.00 |     30 |
| SMITH | CLERK   |  800.00 |     20 |
| ADAMS | CLERK   | 1100.00 |     20 |
| JAMES | CLERK   |  950.00 |     30 |
+-------+---------+---------+--------+
5 rows in set (0.00 sec)

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

cpp 复制代码 
mysql> select ename, sal, deptno from emp where sal > all(select sal from emp where deptno = 30);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| JONES | 2975.00 |     20 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.00 sec)
  • 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）
cpp 复制代码 
mysql> select ename, sal, deptno from emp where sal > any(select sal from emp where deptno = 30);
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| ALLEN  | 1600.00 |     30 |
| WARD   | 1250.00 |     30 |
| JONES  | 2975.00 |     20 |
| MARTIN | 1250.00 |     30 |
| BLAKE  | 2850.00 |     30 |
| CLARK  | 2450.00 |     10 |
| SCOTT  | 3000.00 |     20 |
| KING   | 5000.00 |     10 |
| TURNER | 1500.00 |     30 |
| ADAMS  | 1100.00 |     20 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
12 rows in set (0.00 sec)

💛4.3 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

  • 查询和SMITH的部门和岗位完全相同的所有雇员，不含SMITH本人
cpp 复制代码 
mysql> select * from emp where (deptno, job) = (select deptno, job from emp where ename = 'SMITH') andename <> 'SMITH';
+--------+-------+-------+------+---------------------+---------+------+--------+
| empno  | ename | job   | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-------+------+---------------------+---------+------+--------+
| 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
+--------+-------+-------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)

💚4.4 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。

  • 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

关于临时表中的内容（avg(sal)， deptno）的别名问题

像where emp.sal > tmp.avg(sal)这样的写法（假设没有设置别名asal）是错误的。

在外部查询的连接条件emp.deptno = tmp.deptno部分可能会导致语法混淆，最好设置别名

cpp 复制代码 
mysql> select avg(sal) asal, deptno dt from emp group by deptno;
+-------------+------+
| asal        | dt   |
+-------------+------+
| 2916.666667 |   10 |
| 2175.000000 |   20 |
| 1566.666667 |   30 |
+-------------+------+
3 rows in set (0.00 sec)

// 运用临时表
mysql> select ename, deptno, sal, asal 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     | asal        |
+-------+--------+---------+-------------+
| KING  |     10 | 5000.00 | 2916.666667 |
| JONES |     20 | 2975.00 | 2175.000000 |
| SCOTT |     20 | 3000.00 | 2175.000000 |
| FORD  |     20 | 3000.00 | 2175.000000 |
| ALLEN |     30 | 1600.00 | 1566.666667 |
| BLAKE |     30 | 2850.00 | 1566.666667 |
+-------+--------+---------+-------------+
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
cpp 复制代码 
// 临时表 tmp 内容
mysql> select max(sal) ms, deptno dt from emp group by deptno;
+---------+------+
| ms      | dt   |
+---------+------+
| 5000.00 |   10 |
| 3000.00 |   20 |
| 2850.00 |   30 |
+---------+------+
3 rows in set (0.00 sec)

// 运用临时表
mysql> select ename, sal, deptno, ms from emp, (select max(sal) ms, deptno dt from emp group by deptno)tmp where deptno = dt and sal = 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
cpp 复制代码 
// 方法1：使用多表
mysql> 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;
+------------+--------+----------+--------------+
| dname      | deptno | loc      | 部门人数     |
+------------+--------+----------+--------------+
| ACCOUNTING |     10 | NEW YORK |            3 |
| RESEARCH   |     20 | DALLAS   |            5 |
| SALES      |     30 | CHICAGO  |            6 |
+------------+--------+----------+--------------+
3 rows in set (0.00 sec)`V`

// 方法2：使用子查询
mysql> select dept.deptno, dname, mycnt, loc from dept, (select count(*) mycnt, deptno from emp group by deptno) tmp where dept.deptno = tmp.deptno;
+--------+------------+-------+----------+
| deptno | dname      | mycnt | loc      |
+--------+------------+-------+----------+
|     10 | ACCOUNTING |     3 | NEW YORK |
|     20 | RESEARCH   |     5 | DALLAS   |
|     30 | SALES      |     6 | CHICAGO  |
+--------+------------+-------+----------+
3 rows in set (0.00 sec)

💙4.5 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

4.5.1 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。

  • 将工资大于2500或职位是MANAGER的人找出来
cpp 复制代码 
mysql> select ename, sal, job from emp where sal > 2500 union 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   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
6 rows in set (0.00 sec)

4.5.3 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。

  • 将工资大于25000或职位是MANAGER的人找出来
cpp 复制代码 
mysql> select ename, sal, job from emp where sal > 200 union all
    -> select ename, sal, job from emp where job = 'MANAGER';
+--------+---------+-----------+
| ename  | sal     | job       |
+--------+---------+-----------+
| SMITH  |  800.00 | CLERK     |
| ALLEN  | 1600.00 | SALESMAN  |
| WARD   | 1250.00 | SALESMAN  |
| JONES  | 2975.00 | MANAGER   |
| MARTIN | 1250.00 | SALESMAN  |
| BLAKE  | 2850.00 | MANAGER   |
| CLARK  | 2450.00 | MANAGER   |
| SCOTT  | 3000.00 | ANALYST   |
| KING   | 5000.00 | PRESIDENT |
| TURNER | 1500.00 | SALESMAN  |
| ADAMS  | 1100.00 | CLERK     |
| JAMES  |  950.00 | CLERK     |
| FORD   | 3000.00 | ANALYST   |
| MILLER | 1300.00 | CLERK     |
| JONES  | 2975.00 | MANAGER   |
| BLAKE  | 2850.00 | MANAGER   |
| CLARK  | 2450.00 | MANAGER   |
+--------+---------+-----------+
17 rows in set (0.00 sec)

👥总结

本篇博文对 【MySQL】复合查询 做了一个较为详细的介绍，不知道对你有没有帮助呢

觉得博主写得还不错的三连支持下吧！会继续努力的~

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