复合查询

# 基本查询

mysql> show dept;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that correspoL server version for the right syntax to use near 'dept' at line 1
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)

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)

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)

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查询工资高于500或岗位为MANAGER的员工，同时要求员工姓名的首字母为大写的J

mysql> select ename, job, sal from emp where (sal>500 or job='MANAGER') and (ename like 'J%');
+-------+---------+---------+
| ename | job     | sal     |
+-------+---------+---------+
| JONES | MANAGER | 2975.00 |
| JAMES | CLERK   |  950.00 |
+-------+---------+---------+
2 rows in set (0.00 sec)

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查询员工信息，按部门号升序而员工工资降序显示

mysql> select ename,deptno, sal from emp order by deptno asc, sal desc;
+--------+--------+---------+
| ename  | deptno | sal     |
+--------+--------+---------+
| KING   |     10 | 5000.00 |
| CLARK  |     10 | 2450.00 |
| MILLER |     10 | 1300.00 |
| SCOTT  |     20 | 3000.00 |
| FORD   |     20 | 3000.00 |
| JONES  |     20 | 2975.00 |
| ADAMS  |     20 | 1100.00 |
| SMITH  |     20 |  800.00 |
| BLAKE  |     30 | 2850.00 |
| ALLEN  |     30 | 1600.00 |
| TURNER |     30 | 1500.00 |
| WARD   |     30 | 1250.00 |
| MARTIN |     30 | 1250.00 |
| JAMES  |     30 |  950.00 |
+--------+--------+---------+
14 rows in set (0.00 sec)

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查询员工信息，按年薪降序显示

mysql> select ename, (sal*12+ifnull(0, comm)) 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  | 19200.00 |
| TURNER | 18000.00 |
| MILLER | 15600.00 |
| WARD   | 15000.00 |
| MARTIN | 15000.00 |
| ADAMS  | 13200.00 |
| JAMES  | 11400.00 |
| SMITH  |  9600.00 |
+--------+----------+
14 rows in set (0.00 sec)

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查询工资最高的员工的姓名和岗位

mysql> select ename, sal from emp where sal=(select max(sal) from emp);
+-------+---------+
| ename | sal     |
+-------+---------+
| KING  | 5000.00 |
+-------+---------+

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查询工资高于平均工资的员工

mysql> 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 |
+-------+---------+

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查询每个部门的平均工资和最高工资

mysql> select deptno, avg(sal), max(sal) from emp group by deptno order by deptno;
+--------+-------------+----------+
| deptno | avg(sal)    | max(sal) |
+--------+-------------+----------+
|     10 | 2916.666667 |  5000.00 |
|     20 | 2175.000000 |  3000.00 |
|     30 | 1566.666667 |  2850.00 |
+--------+-------------+----------+
3 rows in set (0.01 sec)

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查询平均工资低于2000的部门号和它的平均工资

mysql> select deptno, avg(sal) from emp group by deptno having avg(sal)<2000;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)

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查询每种岗位的雇员总数和平均工资

mysql> select job, count(job), avg(sal)from emp group by job;
+-----------+------------+-------------+
| job       | count(job) | avg(sal)    |
+-----------+------------+-------------+
| CLERK     |          4 | 1037.500000 |
| SALESMAN  |          4 | 1400.000000 |
| MANAGER   |          3 | 2758.333333 |
| ANALYST   |          2 | 3000.000000 |
| PRESIDENT |          1 | 5000.000000 |
+-----------+------------+-------------+

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# 多表查询

不同的表进行笛卡尔积拼接为一张表

上面的基础查询都是在一张表的基础上进行的查询，而实际开发中往往需要将多张表关联起来进行查询，这就叫做多表查询。
在进行多表查询时，只需要将多张表的表名依次放到from子句之后，用逗号隔开即可，这时MySQL将会对给定的这多张表取笛卡尔积，作为多表查询的初始数据源。
多表查询的本质，就是对给定的多张表取笛卡尔积，然后在笛卡尔积中进行查询。
多张表中可能会存在相同的列名，这时在选中列名时需要通过表名.列名的方式进行指明

通过多表查询，将emp和dept结合为一张表：

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

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显示部门号为10的部门名、员工名和员工工资

由于部门名只有部门表中才有，而员工名和员工工资只有员工表中才有，因此需要同时使用员工表和部门表进行多表查询，在where子句中指明筛选条件为员工的部门号等于部门编号，并且部门号为10的记录:

mysql> select emp.ename,dept.dname,emp.sal from emp, dept where emp.deptno=dept.deptno and emp.deptno=10;
+--------+------------+---------+
| ename  | dname      | sal     |
+--------+------------+---------+
| CLARK  | ACCOUNTING | 2450.00 |
| KING   | ACCOUNTING | 5000.00 |
| MILLER | ACCOUNTING | 1300.00 |
+--------+------------+---------+

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显示各个员工的姓名、工资和工资级别

由于员工名和工资只有员工表中才有，而工资级别只有工资等级表中才有，因此需要同时使用员工表和工资等级表进行多表查询，在where子句中指明筛选条件为员工的工资在losal和hisal之间的记录:

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

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# 自连接

同一张表进行笛卡尔积拼接为一张表

自连接是指在同一张表进行连接查询，也就是说我们不仅可以取不同表的笛卡尔积，也可以对同一张表取笛卡尔积。
如果一张表中的某个字段能够将表中的多条记录关联起来，那么就可以通过自连接将表中通过该字段关联的记录组合起来。
由于自连接是对同一张表取笛卡尔积，因此在自连接时至少需要给一张表取别名，否则无法区分这两张表中的列。

显示员工FORD的上级领导的编号和姓名

解决该问题可以使用子查询，先对员工表进行查询得到FORD的领导的编号，然后再根据领导的编号对员工表进行查询得到FORD领导的姓名。如下：

方式1：

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

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方式2：

mysql> select e2.ename, e2.empno from emp e1, emp e2 where e1.ename='FORD' and e1.mgr=e2.empno;
+-------+--------+
| ename | empno  |
+-------+--------+
| JONES | 007566 |
+-------+--------+
1 row in set (0.00 sec)

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# 子查询

子查询是指嵌入在其他SQL语句中的查询语句，也叫嵌套查询。
子查询可分为单行子查询、多行子查询、多列子查询，以及在from子句中使用的子查询

# 单行子查询

单行子查询，是指返回单行单列数据的子查询。

显示SMITH同一部门的员工

mysql> select * from emp where deptno=(select deptno from emp where ename='SMITH') and ename!='SMITH';
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 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 |
+--------+-------+---------+------+---------------------+---------+------+--------+
4 rows in set (0.00 sec)

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# 多行子查询

多行子查询是指返回多行单列数据的子查询。

in关键字：in关键字用于判断一个值是否在指定的子查询中，如果存在则返回true，否则返回false。
any关键字：any关键字用于判断一个值是否在指定的子查询中，如果存在则返回true，否则返回false。
all关键字：all关键字用于判断一个值是否在指定的子查询中，如果存在则返回true，否则返回false。

显示和10号部门的工作岗位相同的员工的名字、岗位、工资和部门号，但是不包含10号部门的员工

mysql> select ename, job, sal, deptno from emp where job in (select job from emp where deptno=10);
+--------+-----------+---------+--------+
| ename  | job       | sal     | deptno |
+--------+-----------+---------+--------+
| SMITH  | CLERK     |  800.00 |     20 |
| JONES  | MANAGER   | 2975.00 |     20 |
| BLAKE  | MANAGER   | 2850.00 |     30 |
| CLARK  | MANAGER   | 2450.00 |     10 |
| KING   | PRESIDENT | 5000.00 |     10 |
| ADAMS  | CLERK     | 1100.00 |     20 |
| JAMES  | CLERK     |  950.00 |     30 |
| MILLER | CLERK     | 1300.00 |     10 |
+--------+-----------+---------+--------+
8 rows in set (0.00 sec)

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显示工资比30号部门的所有员工的工资高的员工的姓名、工资和部门号

mysql> select ename, job, sal from emp where sal>all (select max(sal) from emp where deptno=30);
+-------+-----------+---------+
| ename | job       | sal     |
+-------+-----------+---------+
| JONES | MANAGER   | 2975.00 |
| SCOTT | ANALYST   | 3000.00 |
| KING  | PRESIDENT | 5000.00 |
| FORD  | ANALYST   | 3000.00 |
+-------+-----------+---------+
4 rows in set (0.00 sec)

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显示工资比30号部门的任意员工的工资高的员工的姓名、工资和部门号，包含30号部门的员工

mysql> select ename, sal, job, deptno from emp where sal > any(select sal from emp where deptno=30);
+--------+---------+-----------+--------+
| ename  | sal     | job       | deptno |
+--------+---------+-----------+--------+
| ALLEN  | 1600.00 | SALESMAN  |     30 |
| WARD   | 1250.00 | SALESMAN  |     30 |
| JONES  | 2975.00 | MANAGER   |     20 |
| MARTIN | 1250.00 | SALESMAN  |     30 |
| BLAKE  | 2850.00 | MANAGER   |     30 |
| CLARK  | 2450.00 | MANAGER   |     10 |
| SCOTT  | 3000.00 | ANALYST   |     20 |
| KING   | 5000.00 | PRESIDENT |     10 |
| TURNER | 1500.00 | SALESMAN  |     30 |
| ADAMS  | 1100.00 | CLERK     |     20 |
| FORD   | 3000.00 | ANALYST   |     20 |
| MILLER | 1300.00 | CLERK     |     10 |
+--------+---------+-----------+--------+
12 rows in set (0.00 sec)

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# 多列子查询

多列子查询是指返回多列多行的子查询。

多列子查询得到的结果是多列数据，在比较多列数据时需要将待比较的多个列用圆括号括起来。
多列子查询返回的如果是多行数据，在筛选数据时也可以使用in、all和any关键字。

显示和SMITH的部门和岗位完全相同的员工，不包含SMITH本人

mysql> select * from emp where (job, deptno)=(select job, deptno from emp where ename='SMITH') and ename!='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)

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# from子句中使用的子查询

from子句中使用的子查询是指在from子句中使用的子查询。

子查询语句不仅可以出现在where子句中，也可以出现在from子句中。
子查询语句出现from子句中，其查询结果将会被当作一个临时表使用。
在from子句中使用子查询时，必须给子查询得到的临时表取一个别名，否则查询将会出错。

显示每个高于自己部门平均工资的员工的姓名、部门、工资和部门的平均工资

mysql> select emp.ename, emp.sal, emp.deptno, temp.avgsal from emp,(select deptno, avg(sal) as avgsal from emp grouup by deptno) temp where emp.deptno=temp.deptno and emp.sal>temp.avgsal;
+-------+---------+--------+-------------+
| ename | sal     | deptno | avgsal      |
+-------+---------+--------+-------------+
| ALLEN | 1600.00 |     30 | 1566.666667 |
| JONES | 2975.00 |     20 | 2175.000000 |
| BLAKE | 2850.00 |     30 | 1566.666667 |
| SCOTT | 3000.00 |     20 | 2175.000000 |
| KING  | 5000.00 |     10 | 2916.666667 |
| FORD  | 3000.00 |     20 | 2175.000000 |
+-------+---------+--------+-------------+
6 rows in set (0.00 sec)

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显示每个部门工资最高的员工的姓名、工资、部门和部门的最高工资

mysql> select emp.ename, emp.sal, emp.deptno, temp.maxsal from emp, (select deptno,  max(sal) as maxsal from emp grroup by deptno) temp where emp.deptno=temp.deptno and emp.sal=maxsal;
+-------+---------+--------+---------+
| ename | sal     | deptno | maxsal  |
+-------+---------+--------+---------+
| 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)

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显示每个部门的部门名、部门编号、所在地址和人员数量

mysql> select * from dept, (select deptno, count(ename) from emp group by deptno) temp where dept.deptno=temp.deptn
no;
+--------+------------+----------+--------+--------------+
| deptno | dname      | loc      | deptno | count(ename) |
+--------+------------+----------+--------+--------------+
|     10 | ACCOUNTING | NEW YORK |     10 |            3 |
|     20 | RESEARCH   | DALLAS   |     20 |            5 |
|     30 | SALES      | CHICAGO  |     30 |            6 |
+--------+------------+----------+--------+--------------+
3 rows in set (0.00 sec)

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# 合并查询

合并查询，是指将多个查询结果进行合并，可使用的操作符有union和union all。

union用于取得两个查询结果的并集，union会自动去掉结果集中的重复行。
union all也用于取得两个查询结果的并集，但union all不会去掉结果集中的重复行。
待合并的两个查询结果的列的数量必须一致，否则无法合并。

上次更新: 2025/03/14, 11:20:46

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