1.首先准备两张表
部门表:
员工表:
以下我们就对这两张表进行不同的连接操作
1.内连接
作用: 查询两张表的共有部分
语句:select from tablea a inner join tableb b on a.key = b.key
示例:select * from employee e inner join department d on e.dep_id = d.id;
结果显示:通过这个查找的方法,我们没有查到id为8的数据
2.左连接
作用:把左边表的内容全部查出,右边表只查出满足条件的记录
语句:select from tablea a left join tableb b on a.key = b.key
示例:select * from employee e left join department d on e.dep_id = d.id;
结果显示:
3.右连接
作用:把右边表的内容全部查出,左边表只查出满足条件的记录
语句:select from tablea a left join tableb b on a.key = b.key
示例:select * from employee e right join department d on e.dep_id = d.id;
结果显示:
4.查询左表独有数据
作用:查询a的独有数据
语句:select from tablea a left join tableb b on a.key = b.key where b.key is null
示例:select * from employee e left join department d on e.dep_id = d.id where d.id is null;
结果显示:
5.查询右表独有数据
作用:查询b的独有数据
语句:select from tablea a right join tableb b on a.key = b.key where a.key is null
示例:select * from employee e right join department d on e.dep_id = d.id where e.id is null;
结果显示:
6.全连接
作用:查询两个表的全部信息
语句:select from tablea a full outter join tableb b on a.key = b.key
注:mysql 默认不支持此种写法 oracle支持 可以使用将左连接与右连接结合起来作为全连接
示例:
select * from employee e left join department d on e.dep_id = d.id union select * from employee e right join department d on e.dep_id = d.id
结果显示:
7.查询左右表各自的独有的数据
作用:查询a和b各自的独有的数据
语句:select from tablea a full outter join tableb b on a.key = b.key where a.key = null or b.key=null
示例:
select * from employee e left join department d on e.dep_id = d.id where d.id is null union select * from employee e right join department d on e.dep_id = d.id where e.dep_id is null
结果显示:
总结
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