left join、right join、inner join的区别
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举例如下:
表A记录
表B记录
1、left join(左联接) 返回包括左表中的所有记录和右表中联结字段相等的记录
sql语句:
select * from A left join B on A.aID=B.bID
结果如下:
aID
aNum
bID
bNum
1
a20050111
1
2006032401
2
a20050112
2
2006032402
3
a20050113
3
2006032403
4
a20050114
4
2006032404
5
a20050115
null
null
(所影响的行数为5)
说明:left join 是以A表为基础的,A可以看成是左表,B看成是右表,left join是以左表为准的。
换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).
B表记录不足的地方均为NULL.
2、right join(右联接) 返回包括右表中的所有记录和左表中联结字段相等的记录
sql语句如下:
select * from Aright join B on A.aID = B.bID
结果:
aID
aNum
bID
bNum
1
a20050111
1
2006032401
2
a20050112
2
2006032402
3
a20050113
3
2006032403
4
a20050114
4
2006032404
null
null
8
2006032408
(所影响的行数为5)
说明:和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
3、inner join(等值连接) 只返回两个表中联结字段相等的行
sql语句:
select * from Ainnerjoin B on A.aID = B.bID
结果:
aID
aNum
bID
bNum
1
a20050111
1
2006032401
2
a20050112
2
2006032402
3
a20050113
3
2006032403
4
a20050114
4
2006032404
说明:这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.
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