【MySQL】表的内外连接

一、内连接

内连接就是利用where子句对两种表形成的笛卡尔积进行筛选，就是在from中把表连接号然后利用where条件筛选。

语法

select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件;

1.1 显示SMITH的名字和部门名称

# 要用到的两张表
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 dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.00 sec)

• 通过笛卡尔积筛选

mysql> select ename, dname from emp t1, dept t2 where t1.deptno=t2.deptno and t1.ename='SMITH';
+-------+----------+
| ename | dname    |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)

• 标准的内连接的写法

mysql> select ename, dname from emp t1 inner join dept t2 on t1.deptno=t2.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)

二、外连接

2.1 左外连接

如果联合查询，左侧的表完全显示就是左外连接。

语法：

select 字段名 from 表名1 left join 表名2 on 连接条件;

现在有两个表结构：

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)

2.1.1 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

可以看到这两张表并不是完全吻合的，如果这里用内连接，未在两个表中同时出现的id字段将不会被筛选出来：

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

• 左外连接

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)

2.2 右外连接

如果联合查询，右侧的表完全显示我们就说是右外连接。

语法：

select 字段 from 表名1 right join 表名2 on 连接条件;

2.2.1 对stu表和exam表联合查询，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

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)

2.2.2 列出部门名称和这些部门的员工信息，同时列出没有员工的部门

mysql> select dname, ename, dept.deptno from emp right join dept on dept.deptno=emp.deptno order by dept.deptno ASC;
+------------+--------+--------+
| dname      | ename  | deptno |
+------------+--------+--------+
| ACCOUNTING | KING   |     10 |
| ACCOUNTING | CLARK  |     10 |
| ACCOUNTING | MILLER |     10 |
| RESEARCH   | FORD   |     20 |
| RESEARCH   | JONES  |     20 |
| RESEARCH   | SMITH  |     20 |
| RESEARCH   | ADAMS  |     20 |
| RESEARCH   | SCOTT  |     20 |
| SALES      | BLAKE  |     30 |
| SALES      | WARD   |     30 |
| SALES      | TURNER |     30 |
| SALES      | MARTIN |     30 |
| SALES      | ALLEN  |     30 |
| SALES      | JAMES  |     30 |
| OPERATIONS | NULL   |     40 |
+------------+--------+--------+
15 rows in set (0.00 sec)