【MySQL】表的内联和外联_mysql 内联

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本篇主题：【MySQL】表的内联和外联
发布时间：2025.2.24
隶属专栏：MySQL

目录

• 内连接
• • 语法
  • 案例
• 外连接
• • 左外连接
  • • 语法
    • 案例
  • 右外连接
  • • 语法
    • 案例
  • 练习
  • • 运用左外连接
    • 运用右外连接

表的连接分为内连和外连

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，我们前面文章的查询都是内连接，也是在开发过程中使用的最多的连接查询。

语法

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

案例

1. 显示SMITH的名字和部门名称

用之前的方法

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

用标准内连接

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

两种方式混合使用，更好的区分笛卡尔积和筛选条件

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

外连接

外连接分为左外连接和右外连接

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

案例

前期准备：

mysql> create table stu (id int, name varchar(30));-- 学生表Query OK, 0 rows affected (0.03 sec)mysql> desc stu;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || name | varchar(30) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> insert into stu values(1,\'jack\'),(2,\'tom\'),(3,\'kity\'),(4,\'nono\');Query OK, 4 rows affected (0.01 sec)Records: 4 Duplicates: 0 Warnings: 0mysql> select * from stu;+------+------+| id | name |+------+------+| 1 | jack || 2 | tom || 3 | kity || 4 | nono |+------+------+4 rows in set (0.00 sec)mysql> create table exam (id int, grade int); -- 成绩表Query OK, 0 rows affected (0.02 sec)mysql> desc exam;+-------+---------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || grade | int(11) | YES | | NULL | |+-------+---------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> insert into exam values(1, 56),(2,76),(11, 8);Query OK, 3 rows affected (0.00 sec)Records: 3 Duplicates: 0 Warnings: 0mysql> select * from exam;+------+-------+| id | grade |+------+-------+| 1 | 56 || 2 | 76 || 11 | 8 |+------+-------+3 rows in set (0.00 sec)

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

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；

案例

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)

练习

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

运用左外连接

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

运用右外连接

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

⚠️ 写在最后：以上内容是我在学习以后得一些总结和概括，如有错误或者需要补充的地方欢迎各位大佬评论或者私信我交流！！！