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oracle中left join和right join的区别浅谈

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2016-06-07 17:56:001126browse

oracle中left join和right join的区别浅谈,需要的朋友可以参考一下

通俗的讲:

A left join B 的连接的记录数与A表的记录数同

A right join B 的连接的记录数与B表的记录数同

A left join B 等价B right join A

table A:

Field_K, Field_A

1 a

3 b

4 c

table B:

Field_K, Field_B

1 x

2 y

4 z

select a.Field_K, a.Field_A, b.Field_K, b.Field_B

from a left join b on a.Field_K=b.Field_K

Field_K Field_A Field_K Field_B

---------- ---------- ---------- ----------

1 a 1 x

3 b NULL NULL

4 c 4 z

select a.Field_K, a.Field_A, b.Field_K, b.Field_B

from a right join b on a.Field_K=b.Field_K

Field_K Field_A Field_K Field_B

---------- ---------- ---------- ----------

1 a 1 x

NULL NULL 2 y

4 c 4 z --

举个例子:

假设a表和b表的数据是这样的。

a b

id name  id stock 

1  a 1 15

2 b 2 50

3 c  

select * from a inner join b on a.id=b.id

这个语法是连接查询中的内连接,它产生的结果是

两个表相匹配的记录出现在结果列表中。

根据上面的表,出现的结果是这样的

a.id name b.id stock

1   a 1 15

2 b 2 50

----------------------------

select * from a,b where a.id=b.id

这个语法是内连接的另外一种写法,其执行结果与inner join 一样

--------------------------------

select * from a left/right join b on a.id=b.id

这个是外连接语法中的左外连接或右外连接

如果是左外连接的话,它将显示a表的所有记录,

select a.*,b.* from a left join b on a.id=b.id

查询的结果是这样的:

a.id name b.id stock

1   a 1 15

2 b 2 50

3 c null null 

--------------------------------------------

如果是右外连接的话,它将显示b表的所有记录,

select a.*,b.* from a right join b on a.id=b.id

查询的结果是这样的:

a.id name b.id stock

1   a 1 15

2 b 2 50

--

select a.*,b.* from a left join b on a.k = b.k

select a.*,b.* from a left outer join b on a.k =b.k

----------上面两种一样left join是left outer join的简写

select a.*,b.* from a left inner join b on a.k = b.k

没有这种写法,错误的语句.

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