sql之inner join、left join、right join、full join的区别
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1、join(内连接):用于两个或多个表中的列之间的关系,只返回表中联结字段相等的行
selec * from table1 t1,table2 t2 ,table3 t3…
where t1.id=t2.id=t3.id
2、inner join(内连接) :INNER JOIN 与 JOIN 是相同的
selec * from table1 t1
inner join table2 t2
on 条件 and 条件
3、left join(左连接) :从左表(t1)那里返回所有的行,即使在右表(t2)中没有匹配的行,右表中不存在的均为null
select * from table1 t1
left join table2 t2 on 条件 and 条件
left join table3 t3 on 条件 and 条件
4、right join(右连接): 从右表(t2)那里返回所有的行,即使在左表(t1)中没有匹配的行,左表中不存在的均为null
select * from table1 t1
right join table2 t2
on条件 and 条件
5、full join(全连接):只要关联的表中关键字存在值,就会返回行
select * from table1 t1
full join table2 t2
on 条件 and 条件
----------------------举例如下------------------------------------------------------------------------------------------------------------------------
表:Persons
表:Orders
1.join 实例如下:
select * from Persons p,Orders o where p.id_P= o.id_P
结果如下:
2. inner join 实例如下:
select * from Persons p inner join Orders o on p.id_P= o.id_P
结果如下:
d_PNamephoneAddressId_OOrderNoId_P1张三18059861111厦门思明1OR201712050111张三18059861111厦门思明2OR201712050213王二18059863333广州佛山3OR201712050333王二18059863333广州佛山4OR20171205043
3.left join实例如下:
select * from Persons p left join Orders o on p.id_P= o.id_P
结果如下:
4.right join实例如下:
select * from Persons p right join Orders o on p.id_P= o.id_P
结果如下:
5.full join实例如下:
select * from Persons p full join Orders o on p.id_P= o.id_P
结果如下:
----------------------举例如下------------------------------------------------------------------------------------------------------------------------
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