left join、right join、inner join的区别
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left join 左联接查询: 以左表为基础,执行数据查询,联接表中,联结字段相等的记录,链接字段相等的记录数据做笛卡尔乘法。
right join 右边联接查询: 以右表为基础,执行数据查询,联接表中,联结字段相等的记录,链接字段相等的记录数据做笛卡尔乘法。
inner join 内连接查询: 返回两个表中联结字段相等的数据,同样记录数也是做笛卡尔乘法。
举例说明:
--------------------------------------------
表tab_1记录如下:
ID name
1 OK1
2 OK2
3 OK2
4 ASDF
表tab_2记录如下:
ID name
1 OK1
2 TEST
3 OK2
4 OK2
--------------------------------------------
左连接查找:select * from tab_1 A left join tab_2 B on A.name = B.name
查找结果:
1 OK1 1 OK1
2 OK2 3 OK2
2 OK2 4 OK2
3 OK2 3 OK2
3 OK2 4 OK2
4 ASDF null null
右连接查找:select * from tab_1 A right join tab_2 B on A.name = B.name
查找结果:
1 OK1 1 OK1
null null 2 TEST
2 OK2 3 OK2
3 OK2 3 OK2
2 OK2 4 OK2
3 OK2 4 OK2
内连接查找:select * from tab_1 A inner join tab_2 B on A.name = B.name
查找结果:
1 OK1 1 OK1
2 OK2 3 OK2
2 OK2 4 OK2
3 OK2 3 OK2
3 OK2 4 OK2
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