mysql 之: inner join , left join 和 right join 的区别

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通俗的讲：

A left join B |:连接的记录数与A表的记录数同(显示A表的所有记录 ,B表不存在记录则为NULL)

A right join B |:连接的记录数与B表的记录数同(显示B表的所有记录 ,A表不存在记录则为NULL)

A left join B |: 等价B right join A

A inner join B |: 两个表相匹配的记录出现在查询结果列表中

提示: select * from a,b where a.Field_K=b.Field_K |联表查询 :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 ;

提示 :left join是left outer join的简写 ,查询结果一样;

表结构: ===========================================================================

table A:
Field_K, Field_A

1 a

3 b

4 c

table B:

Field_K, Field_B

1 x

2 y

4 z

操作实例: =============================================================================

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

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

select a.Field_K, a.Field_A, b.Field_K, b.Field_B
from a inner join b on a.Field_K=b.Field_K

  Field_K         Field_A         Field_K         Field_B
  ----------   ----------   ----------   ----------
  1                     a                     1                     x

4 c 4 z

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