五、多表连接查询——外连接

今天药忘吃喽~ 2023-06-22 14:00 12阅读 0赞

## 五、多表连接查询——外连接 ##

### 1.左外连接 ###

简单来说，就是在等值连接的基础上，保留左侧表的独有数据，右表没有的用null替代。

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### 2.右外连接 ###

简单来说，就是在等值连接的基础上，保留右侧表的独有数据，左表没有的用null替代。

select * from student right join student_score on student.student_id=student_score.student_id;

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### 3.全外连接 ###

保留各自表中独特的部分。

Mysql暂不支持全外连接，可采用其它方式替代：使用union关联两个表的结果即全外连接。

**union会整合多个查询结果到一个表中。**

select * from student left join student_score on student.student_id=student_score.student_id
    union
    select * from student right join student_score on student.student_id=student_score.student_id;

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