SQL中join的用法解析（inner join、full out join、left join）

话不多说，先上个对于各种join情况的总结图：

 

 

 

例如有如下关系的数据：

 

 

 

1、inner join

 

 

 

 

select * from Table_a
join Table_b on Table_a.id_a = Table_b.id_b

　　

 

 

 

2.1、full outer join等于full join

 

 

 

 

select * from Table_a
full join Table_b
on Table_a.id_a = Table_b.id_b

　　

 

 

 

2.2、full outer join but a.Key is null or b.Key is null

 

 

 

select * from Table_a
full outer join Table_b
on Table_a.id_a = Table_b.id_b
where Table_a.id_a is null or Table_b.id_b is null

　　

 

 

3.1、left outer join等于left join

 

 

select * from Table_a
left join Table_b on Table_a.id_a = Table_b.id_b

　　

 

 

3.2、left outer join but b.Key is null

 

 

 

select * from Table_a
left outer join Table_b on Table_a.id_a = Table_b.id_b
where Table_b.id_b is null

　　

 

笛卡尔积

select * from Table_a ,Table_b