left join和right join的区别

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