【MySQL】表的内联和外联

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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

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: 0

mysql> 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: 0

mysql> 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！！！