【MySQL】复合查询（子查询/多表查询）

in 关键字

语法
sql 复制代码
select ... from table where coulmn in (select 子查询的结果，有两行及以上);
作用：如果 coulmn 列的数据在子查询结果中，那么条件为真，否则为假

案例：

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号部门的雇员
sql 复制代码
select ename,job,sal,deptno from emp where job in (select distinct job from 
emp where deptno=10) and deptno != 10;
all 关键字

语法
sql 复制代码
select ... from table where coulmn > all (select 子查询的结果，有两行及以上);
select ... from table where coulmn < all (select 子查询的结果，有两行及以上);
作用：如果 coulmn 列的数据比所有的子查询结果都要大(小)（即大(小)于子查询结果的最大(小)值），那么条件为真，否则为假

案例：

显示工资比 30 号部门的所有员工的工资都要高的员工的姓名、工资和部门
sql 复制代码
select ename, sal, deptno from EMP where sal > all(select sal from EMP where 
deptno = 30);
any 关键字

语法
sql 复制代码
select ... from table where coulmn > any (select 子查询的结果，有两行及以上);
select ... from table where coulmn < any (select 子查询的结果，有两行及以上);
作用：只要 coulmn 列的数据有比子查询结果大(小)的，那么条件为真，否则为假

案例：

显示工资只要比 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）
sql 复制代码
select ename, sal, deptno from EMP where sal > any(select sal from EMP where 
deptno=30);

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

sql 复制代码

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

' = ' 也可以换成 ' in '，即把 (deptno, job) 看成一个整体，如果子查询有多列，则匹配所有与 (deptno, job) 相同的列。

在 from 子句中使用子查询

在 MySQL 中，一切皆表，包括 select 查询过程的临时表，我们可以利用这些临时表，与现有的表或其他临时表做笛卡尔积，再从结果做筛选，得到一些有用的信息。

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、部门平均工资

sql 复制代码

// 步骤1；先显示每个部门的平均工资
mysql> select DEPTNO,avg(sal) from EMP group by DEPTNO;
+--------+-------------+
| DEPTNO | avg(sal)    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)

// 步骤二：将上面的临时表与 EMP 表做笛卡尔积
mysql> select * from EMP,(select DEPTNO,avg(sal) 
mysql> from EMP group by DEPTNO) as tmp;
+-------+--------+-----------+------+------------+---------+---------+--------+--------+-------------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | avg(sal)    |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+-------------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     10 | 2916.666667 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     10 | 2916.666667 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     10 | 2916.666667 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     10 | 2916.666667 |
//...(部分)

// 步骤三：过滤掉上面的临时表的无意义的行，即 EMP.DEPTNO != tmp.DEPTNO 的行
mysql> select * from EMP,(select DEPTNO,avg(sal) 
mysql> from EMP group by DEPTNO) as tmp where EMP.DEPTNO = tmp.DEPTNO;
+-------+--------+-----------+------+------------+---------+---------+--------+--------+-------------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | avg(sal)    |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+-------------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | 2175.000000 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | 1566.666667 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | 1566.666667 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | 2175.000000 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | 1566.666667 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | 1566.666667 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | 2916.666667 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | 2175.000000 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | 2916.666667 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | 1566.666667 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | 2175.000000 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | 1566.666667 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | 2175.000000 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | 2916.666667 |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+-------------+
14 rows in set (0.00 sec)

// 步骤四：利用上面的临时表，筛选出高于自己部门平均工资的员工的姓名、部门、工资、部门平均工资
mysql> select ENAME,SAL,EMP.DEPTNO,avg 
mysql> from EMP,(select DEPTNO,avg(sal) as avg from EMP group by DEPTNO) as tmp 
mysql> where EMP.DEPTNO = tmp.DEPTNO and EMP.SAL > avg;
+-------+---------+--------+-------------+
| ENAME | SAL     | DEPTNO | avg         |
+-------+---------+--------+-------------+
| 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)

上面的最终结果的表还可以作为临时表，与其他表做笛卡尔积......

合并查询

合并多个 select 的执行结果，可以使用集合操作符 union，union all

语法：

sql 复制代码

select ... union select ...;
select ... union all select ...;

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

sql 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   |
+-------+---------+-----------+

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   |
+-------+---------+-----------+

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。

语法

复制代码

select ... from table_name1,table_name2,...;

笛卡尔积

案例：显示雇员名、雇员工资以及所在部门的名字。雇员名、雇员工资来自 EMP 表，部门的名字来自 DEPT 表。

复制代码

mysql> show tables;
+-----------------+
| Tables_in_scott |
+-----------------+
| BONUS           |
| DEPT            |
| EMP             |
| SALGRADE        |
+-----------------+
4 rows in set (0.00 sec)

mysql> select * from EMP;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  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 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows 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)

mysql> select * from EMP,DEPT;
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     10 | ACCOUNTING | NEW YORK |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     20 | RESEARCH   | DALLAS   |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |     40 | OPERATIONS | BOSTON   |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     10 | ACCOUNTING | NEW YORK |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     20 | RESEARCH   | DALLAS   |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |     40 | OPERATIONS | BOSTON   |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     10 | ACCOUNTING | NEW YORK |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     20 | RESEARCH   | DALLAS   |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |     40 | OPERATIONS | BOSTON   |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     10 | ACCOUNTING | NEW YORK |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     20 | RESEARCH   | DALLAS   |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |     40 | OPERATIONS | BOSTON   |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
56 rows in set (0.00 sec)

观察发现：同时查询两张表（EMP 和 DEPT）时，是把 EMP 的每一行与 DEPT的每一行都组合起来，然后显示。这种穷举组合的方式称为两张表的笛卡尔积。

但这种组合方式组合后的一些行是没有意义的，比如：

复制代码

+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | DEPTNO | DNAME      | LOC      |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |

史密斯在表中出现了四次，而上表的第二行才是我们想要的，所以我们要对 EMP 和 DEPT 合成的一张表进行条件筛选：

复制代码

mysql> select ENAME,SAL,DNAME from EMP,DEPT where EMP.DEPTNO = DEPT.DEPTNO;
+--------+---------+------------+
| ENAME  | SAL     | DNAME      |
+--------+---------+------------+
| CLARK  | 2450.00 | ACCOUNTING |
| KING   | 5000.00 | ACCOUNTING |
| MILLER | 1300.00 | ACCOUNTING |
| SMITH  |  800.00 | RESEARCH   |
| JONES  | 2975.00 | RESEARCH   |
| SCOTT  | 3000.00 | RESEARCH   |
| ADAMS  | 1100.00 | RESEARCH   |
| FORD   | 3000.00 | RESEARCH   |
| ALLEN  | 1600.00 | SALES      |
| WARD   | 1250.00 | SALES      |
| MARTIN | 1250.00 | SALES      |
| BLAKE  | 2850.00 | SALES      |
| TURNER | 1500.00 | SALES      |
| JAMES  |  950.00 | SALES      |
+--------+---------+------------+
14 rows in set (0.00 sec)

综合案例

显示部门号为10的部门名，员工名和工资
sql 复制代码
select ename, sal,dname from EMP, DEPT where EMP.deptno=DEPT.deptno and DEPT.deptno = 10;
显示各个员工的姓名，工资，及工资级别
sql 复制代码
select ename, sal, grade from EMP, SALGRADE where EMP.sal between losal and hisal;

内外连接

内连接

内连接实际上就是利用 where 子句对两种表形成的笛卡儿积进行筛选，我们前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询。

语法

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select 列名1,列名2,列名3... from 表1 inner join 表2 on 连接条件 and 其他条件；

案例：显示SMITH的名字和部门名称

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-- 用前面的写法
select ename, dname from EMP, DEPT 
where EMP.deptno=DEPT.deptno and 
ename='SMITH';

-- 用标准的内连接写法
select ename, dname from EMP inner join DEPT 
on EMP.deptno=DEPT.deptno and 
ename='SMITH';

外连接

外连接分为左外连接和右外连接

左外连接

如果对两张表做笛卡尔积，左侧的表完全显示我们就说是左外连接。

语法

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select 列名1,列名2,列名3... from 表名1 left join 表名2 on 连接条件;
// 表名1 完全显示

案例

查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来

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-- 建两张表
create table stu (id int, name varchar(30)); -- 学生表
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
create table exam (id int, grade int); -- 成绩表
insert into exam values(1, 56),(2,76),(11, 8);

mysql> select * from stu;
+------+------+
| id   | name |
+------+------+
|    1 | jack |
|    2 | tom  |
|    3 | kity |
|    4 | nono |
+------+------+
4 rows in set (0.00 sec)

mysql> select * from exam;
+------+-------+
| id   | grade |
+------+-------+
|    1 |    56 |
|    2 |    76 |
|   11 |     8 |
+------+-------+
3 rows in set (0.00 sec)

接下来我们要对 stu 和 exam 做笛卡尔积，然后对结果表做 where 筛选，筛选条件当然是 stu.id = exam.id ，但是 id 为 3 和 4 的学生没有在 exam 表出现（没有成绩），我们的需求是如果这个学生没有成绩，也要将学生的个人信息显示出来，此时就要用到左外连接：

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mysql> select * from stu left join exam on stu.id=exam.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
|    3 | kity | NULL |  NULL |
|    4 | nono | NULL |  NULL |
+------+------+------+-------+
4 rows in set (0.00 sec)

右外连接

如果对两张表做笛卡尔积，右侧的表完全显示我们就说是右外连接。

语法

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select 列名1,列名2,列名3... from 表名1 right join 表名2 on 连接条件;
// 表名2 完全显示

// 将左外连接的表名顺序颠倒可以实现右外连接
select 列名1,列名2,列名3... from 表名2 left join 表名1 on 连接条件;
// 等价于上面的 SQL

案例

查询所有学生的成绩，把所有的成绩都显示出来，即使这个成绩没有学生与它对应，也要显示出来

sql 复制代码

mysql> select * from stu right join exam on stu.id=exam.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
| NULL | NULL |   11 |     8 |
+------+------+------+-------+
3 rows in set (0.00 sec)

自连接

对同一张表做笛卡尔积，叫做表的自连接

语法

sql 复制代码

select ... from table_name as 重命名1,table_name as 重命名2;

案例：

显示员工FORD的上级领导的编号和姓名

sql 复制代码

mysql> select * from EMP;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  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 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 rows in set (0.00 sec)

方法一：使用子查询

sql 复制代码

select empno,ename from emp where emp.empno=(select mgr from emp where 
ename='FORD');

方法二：使用多表查询（自连接）

sql 复制代码

select leader.empno,leader.ename from emp as leader, emp as worker where 
leader.empno = worker.mgr and worker.ename='FORD';