【MySQL】 表的内外连接

1.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
• 语法：

select 字段 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

//用前面的写法
mysql> select dname from emp,dept where emp.deptno=dept.deptno and ename='SMITH';
+----------+
| dname    |
+----------+
| RESEARCH |
+----------+
1 row in set (0.00 sec)

//用标准的内连接写法
mysql> select dname from emp inner join dept on emp.deptno=dept.deptno where ename='SMITH';
+----------+
| dname    |
+----------+
| RESEARCH |
+----------+
1 row in set (0.00 sec)

2.外连接

• 左外连接

如果联合查询，左侧的表完全显示我们就说是左外连接
语法：
select 字段名 from 表名1 left join 表名2 on 连接条件

• 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；

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