连接查询

1.6.1 什么是连接查询?

在实际开发中，大部分的情况下都不是从单表中查询数据，一般都是多张表联合查询取出最终的结果。

在实际开发中，一般一个业务都会对应多张表，比如:学生和班级，起码两张表。

stuno	stu_name	class_no	classname
1	zs	1	北京大兴区亦庄经济技术开发区第二中学高三1班
1	wer	1	北京大兴区亦庄经济技术开发区第二中学高三1班

学生和班级信息存储到一张表中，结果就像上面一样，数据会存在大量的重复，导致数据的冗余。

1.6.2 连接查询的分类

根据语法出现的年代划分
- SQL92（一些老的DBA可能还在使用这种）
- SQL99（比较新的语法）
根据表的连接方式分类
- 内连接
  - 等值连接
  - 非等值连接
  - 自连接
- 外连接
  - 左连接
  - 右连接
- 全连接（很少使用）

1.6.3 笛卡尔乘积现象

案例：找出每个员工的部门名称，要求显示员工名和部门名。

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)

可以发现要想实现这个功能，我们需要从两个表中分别取出两个数据。

假如我们写一条这样的SQL语句：

select ename, dname from emp , dept;

，最后的结果会是两个字段的笛卡尔乘积，这就是笛卡尔积现象。

即当两个表的筛选未加任何筛选条件时，就会出现上述现象。

mysql> select ename, dname from emp , dept;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | ACCOUNTING |
| SMITH  | RESEARCH   |
| SMITH  | SALES      |
| SMITH  | OPERATIONS |
| ALLEN  | ACCOUNTING |
| ALLEN  | RESEARCH   |
| ALLEN  | SALES      |
| ALLEN  | OPERATIONS |
| WARD   | ACCOUNTING |
| WARD   | RESEARCH   |
| WARD   | SALES      |
| WARD   | OPERATIONS |
| JONES  | ACCOUNTING |
| JONES  | RESEARCH   |
| JONES  | SALES      |
| JONES  | OPERATIONS |
| MARTIN | ACCOUNTING |
| MARTIN | RESEARCH   |
| MARTIN | SALES      |
| MARTIN | OPERATIONS |
| BLAKE  | ACCOUNTING |
| BLAKE  | RESEARCH   |
| BLAKE  | SALES      |
| BLAKE  | OPERATIONS |
| CLARK  | ACCOUNTING |
| CLARK  | RESEARCH   |
| CLARK  | SALES      |
| CLARK  | OPERATIONS |
| SCOTT  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| SCOTT  | SALES      |
| SCOTT  | OPERATIONS |
| KING   | ACCOUNTING |
| KING   | RESEARCH   |
| KING   | SALES      |
| KING   | OPERATIONS |
| TURNER | ACCOUNTING |
| TURNER | RESEARCH   |
| TURNER | SALES      |
| TURNER | OPERATIONS |
| ADAMS  | ACCOUNTING |
| ADAMS  | RESEARCH   |
| ADAMS  | SALES      |
| ADAMS  | OPERATIONS |
| JAMES  | ACCOUNTING |
| JAMES  | RESEARCH   |
| JAMES  | SALES      |
| JAMES  | OPERATIONS |
| FORD   | ACCOUNTING |
| FORD   | RESEARCH   |
| FORD   | SALES      |
| FORD   | OPERATIONS |
| MILLER | ACCOUNTING |
| MILLER | RESEARCH   |
| MILLER | SALES      |
| MILLER | OPERATIONS |
+--------+------------+
56 rows in set (0.00 sec)

此外，补充一点关于表的别名的问题：

select e.ename, d.dname from emp e, dept d;

这样给表起了别名。

表的别名有什么好处：

第一：执行效率高。

第二：可读性好。

若是要完成上面的案例，怎么避免笛卡尔积现象呢？加条件！！！

思考：避免了笛卡尔积现象会减少记录的匹配次数吗？

不会！次数还是56次，只不过显示的是有效记录。

还是考虑上面的案例：

select e.ename, d.dname from emp e, dept d where e.deptno = d.deptno;

这是一种SQL92的写法：

mysql> select e.ename, d.dname from emp e, dept d where e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.01 sec)

1.6.3 内连接

1.等值连接

等值连接是内连接的一种，条件是等量关系。

案例：查询每个员工的部门名称，要求显示员工名和部门名。

SQL92写法（不常用）

mysql> select e.ename, d.dname from emp e, dept d where e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

SQL99（常用）

mysql> select e.ename, d.dname from emp e join dept d on e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

语法：

select ... from ... join ... on... where ...

SQL99语法结构将表连接调节与

where...

数据过滤的条件分离了，这种语法更加清晰。

其中

join

前面省略了一个

inner

，即完整的写法是：

select ... from ... inner join ... on... where ...

2.非等值连接

案例：找出每个员工的工资等级，要求显示员工名，工资，工资等级。

mysql> select ename, sal from emp;
+--------+---------+
| ename  | sal     |
+--------+---------+
| SMITH  |  800.00 |
| ALLEN  | 1600.00 |
| WARD   | 1250.00 |
| JONES  | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE  | 2850.00 |
| CLARK  | 2450.00 |
| SCOTT  | 3000.00 |
| KING   | 5000.00 |
| TURNER | 1500.00 |
| ADAMS  | 1100.00 |
| JAMES  |  950.00 |
| FORD   | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+

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)

语法为：（

inner

可以省略）

mysql> select e.ename, e.sal, s.grade from emp e inner join salgrade s
    -> on
    -> e.sal between s.losal and s.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

案例：找出每个员工的上级领导，显示员工名和对应的上级领导名。

原始数据：

mysql> select empno, ename, mgr from emp;
+-------+--------+------+
| empno | ename  | mgr  |
+-------+--------+------+
|  7369 | SMITH  | 7902 |
|  7499 | ALLEN  | 7698 |
|  7521 | WARD   | 7698 |
|  7566 | JONES  | 7839 |
|  7654 | MARTIN | 7698 |
|  7698 | BLAKE  | 7839 |
|  7782 | CLARK  | 7839 |
|  7788 | SCOTT  | 7566 |
|  7839 | KING   | NULL |
|  7844 | TURNER | 7698 |
|  7876 | ADAMS  | 7788 |
|  7900 | JAMES  | 7698 |
|  7902 | FORD   | 7566 |
|  7934 | MILLER | 7782 |
+-------+--------+------+
14 rows in set (0.00 sec)

原始数据只能找到领导编号，而非领导名。

这既是一个员工表，也能看成一个领导表，我们把这个表起两个别名。

mysql> select
    -> a.ename, b.ename
    -> from
    -> emp a
    -> inner join
    -> emp b
    -> on
    -> a.mgr = b.empno;
+--------+-------+
| ename  | ename |
+--------+-------+
| SMITH  | FORD  |
| ALLEN  | BLAKE |
| WARD   | BLAKE |
| JONES  | KING  |
| MARTIN | BLAKE |
| BLAKE  | KING  |
| CLARK  | KING  |
| SCOTT  | JONES |
| TURNER | BLAKE |
| ADAMS  | SCOTT |
| JAMES  | BLAKE |
| FORD   | JONES |
| MILLER | CLARK |
+--------+-------+
13 rows in set (0.01 sec)

1.6.4 外连接

什么是外连接？和内连接有什么区别？

内连接：假设A表和B表进行内连接，凡是A表和B表能够匹配上的记录都查询出来，这就是内连接。

外连接：假设A表和B表进行链接，那么AB两张表，一张是主表，一张是附表，主要查询主表中的数据，捎带着查询附表，当附表中的数据没有和副表中给的数据匹配上，副表中自动匹配出NULL。

外连接分类：

左（外）连接：表示左边的表是主表
右（外）连接：表示右边的表是主表

左连接有对应的右连接的写法，右连接也有对应的左的写法。

仍是借用1.6.3中的员工上级领导的例子，我们还要求所有员工必须查询出来，也就是king必须查询出来（在内连接中King不会被查询出来）

原内连接的写法： select a.ename, d.ename from emp a inner join emp d on a.mgr = d.empno;
外连接的左外连接的写法： select a.ename, d.ename from emp a left join emp d on a.mgr = d.empno;

mysql> select a.ename, d.ename from emp a inner join emp d on a.mgr = d.empno;
+--------+-------+
| ename  | ename |
+--------+-------+
| SMITH  | FORD  |
| ALLEN  | BLAKE |
| WARD   | BLAKE |
| JONES  | KING  |
| MARTIN | BLAKE |
| BLAKE  | KING  |
| CLARK  | KING  |
| SCOTT  | JONES |
| TURNER | BLAKE |
| ADAMS  | SCOTT |
| JAMES  | BLAKE |
| FORD   | JONES |
| MILLER | CLARK |
+--------+-------+
13 rows in set (0.00 sec)

mysql> select a.ename, d.ename from emp a left join emp d on a.mgr = d.empno;
+--------+-------+
| ename  | ename |
+--------+-------+
| SMITH  | FORD  |
| ALLEN  | BLAKE |
| WARD   | BLAKE |
| JONES  | KING  |
| MARTIN | BLAKE |
| BLAKE  | KING  |
| CLARK  | KING  |
| SCOTT  | JONES |
| KING   | NULL  |
| TURNER | BLAKE |
| ADAMS  | SCOTT |
| JAMES  | BLAKE |
| FORD   | JONES |
| MILLER | CLARK |
+--------+-------+
14 rows in set (0.00 sec)

可以看出左连接的多了一个King。

对应的右连接的写法为：

select a.ename, b.ename from emp b right join emp a on a.mgr = b.empno;

不论是左连接还是右连接，left和right后面都省略了outer关键字。

三种写法对比：

select a.ename, b.ename from emp a inner join emp b on a.mgr = b.empno; ：内连接
select a.ename, b.ename from emp a left outer join emp b on a.mgr = b.empno; ：左外连接
select a.ename, b.ename from emp b right outer join emp a on a.mgr = b.empno; ：右外连接

外连接最主要的特点：主表数据无条件查询出来。

案例：找出哪个部门没有员工。

员工表：

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

部门表：

+--------+------------+----------+
| DEPTNO | DNAME      | LOC      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+

可知，部门表为主表，员工表应该为副表。我们先进行下面的外连接查询：

select d.deptno, d.dname, a.* from dept d left outer join emp a on d.deptno = a.deptno;

mysql> select d.deptno, d.dname, a.* from dept d left outer join emp a on d.deptno = a.deptno;
+--------+------------+-------+--------+-----------+------+------------+---------+---------+--------+
| deptno | dname      | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+--------+------------+-------+--------+-----------+------+------------+---------+---------+--------+
|     10 | ACCOUNTING |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|     10 | ACCOUNTING |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
|     10 | ACCOUNTING |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
|     20 | RESEARCH   |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|     20 | RESEARCH   |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|     20 | RESEARCH   |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
|     20 | RESEARCH   |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|     20 | RESEARCH   |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|     30 | SALES      |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|     30 | SALES      |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|     30 | SALES      |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|     30 | SALES      |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
|     30 | SALES      |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|     30 | SALES      |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|     40 | OPERATIONS |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL |
+--------+------------+-------+--------+-----------+------+------------+---------+---------+--------+

在上面查询基础上，我们还要进一步筛选才能得到无员工的部门，所以我们加上一个

where

条件即可：

select d.deptno, d.dname, a.* from dept d left outer join emp a on d.deptno = a.deptno where empno is null;

mysql> select d.deptno, d.dname, a.* from dept d left outer join emp a on d.deptno = a.deptno where empno is null;
+--------+------------+-------+-------+------+------+----------+------+------+--------+
| deptno | dname      | EMPNO | ENAME | JOB  | MGR  | HIREDATE | SAL  | COMM | DEPTNO |
+--------+------------+-------+-------+------+------+----------+------+------+--------+
|     40 | OPERATIONS |  NULL | NULL  | NULL | NULL | NULL     | NULL | NULL |   NULL |
+--------+------------+-------+-------+------+------+----------+------+------+--------+
1 row in set (0.00 sec)

或者

select d.* from dept d left outer join emp a on d.deptno = a.deptno where empno is null;

mysql> select d.* from dept d left outer join emp a on d.deptno = a.deptno where empno is null;
+--------+------------+--------+
| DEPTNO | DNAME      | LOC    |
+--------+------------+--------+
|     40 | OPERATIONS | BOSTON |
+--------+------------+--------+
1 row in set (0.00 sec)

1.6.5 全连接

两张表全是主表，一般用的很少，所以不予说明。

1.6.6 多表连接查询

案例：找出每一个员工的部门名称以及工资等级。

三个数据分布在三个表中：

emp,dept,salgrade

；

多表连接查询的语法：

select ... from A join B on ... join C on ...

表示：A表和B表先进行表连接，连接之后A表继续和C表连接。

那么案例的表述可以是：

select e.ename, d.dname, s.grade from emp e join dept d on e.deptno = d.deptno join salgrade s on e.sal between s.losal and s.hisal;

mysql> select e.ename, d.dname, s.grade from emp e join dept d on e.deptno = d.deptno join salgrade s on e.sal between s.losal and s.hisal;
+--------+------------+-------+
| ename  | dname      | grade |
+--------+------------+-------+
| SMITH  | RESEARCH   |     1 |
| ALLEN  | SALES      |     3 |
| WARD   | SALES      |     2 |
| JONES  | RESEARCH   |     4 |
| MARTIN | SALES      |     2 |
| BLAKE  | SALES      |     4 |
| CLARK  | ACCOUNTING |     4 |
| SCOTT  | RESEARCH   |     4 |
| KING   | ACCOUNTING |     5 |
| TURNER | SALES      |     3 |
| ADAMS  | RESEARCH   |     1 |
| JAMES  | SALES      |     1 |
| FORD   | RESEARCH   |     4 |
| MILLER | ACCOUNTING |     2 |
+--------+------------+-------+
14 rows in set (0.00 sec)

案例：找出每一个员工的部门名称以及工资等级和上级领导。

额外要用到自连接。最后一个一定要是外连接不然King的数据会丢失。

select e.ename, d.dname, s.grade, e2.ename

from emp e

join dept d on e.deptno = d.deptno

join salgrade s on e.sal between s.losal and s.hisal left

outer join emp e2 on e.mgr = e2.empno;

mysql>  select e.ename, d.dname, s.grade, e2.ename  from emp e join dept d on e.deptno = d.deptno join salgrade s on e.sal between s.losal and s.hisal left outer join emp e2 on e.mgr = e2.empno;
+--------+------------+-------+-------+
| ename  | dname      | grade | ename |
+--------+------------+-------+-------+
| SMITH  | RESEARCH   |     1 | FORD  |
| ALLEN  | SALES      |     3 | BLAKE |
| WARD   | SALES      |     2 | BLAKE |
| JONES  | RESEARCH   |     4 | KING  |
| MARTIN | SALES      |     2 | BLAKE |
| BLAKE  | SALES      |     4 | KING  |
| CLARK  | ACCOUNTING |     4 | KING  |
| SCOTT  | RESEARCH   |     4 | JONES |
| KING   | ACCOUNTING |     5 | NULL  |
| TURNER | SALES      |     3 | BLAKE |
| ADAMS  | RESEARCH   |     1 | SCOTT |
| JAMES  | SALES      |     1 | BLAKE |
| FORD   | RESEARCH   |     4 | JONES |
| MILLER | ACCOUNTING |     2 | CLARK |
+--------+------------+-------+-------+
14 rows in set (0.00 sec)

MySQL连接查询详解：外连接，内连接，全连接连接查询

连接查询

1.6.1 什么是连接查询?

1.6.2 连接查询的分类

1.6.3 笛卡尔乘积现象

1.6.3 内连接

1.等值连接

2.非等值连接

3.自连接

1.6.4 外连接

1.6.5 全连接

1.6.6 多表连接查询

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