sql 内链接 左右连接

inner join(等值链接) 只返回两个表中联结字段相等的行

left join(左联接) 返回包括左表中的全部记录和右表中联结字段相等的记录

right join(右联接) 返回包括右表中的全部记录和左表中联结字段相等的记录

2.操做实例

表A记录以下：
aID               aNum
1                  a20050111
2                  a20050112
3                  a20050113
4                  a20050114
5                  a20050115

表B记录以下:
bID               bName
1                   2006032401
2                  2006032402
3                  2006032403
4                  2006032404
8                  2006032408

实验以下:
1.left join

sql语句以下:
select * from A
left join B
on A.aID = B.bID

结果以下:
aID               aNum                          bID                  bName
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 行）

结果说明:
               left join是以A表的记录为基础的,A能够当作左表,B能够当作右表,left join是以左表为准的.
换句话说,左表(A)的记录将会所有表示出来,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
sql语句以下:
select * from A
right join B
on A.aID = B.bID
结果以下:
aID               aNum                          bID                  bName
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 行）
结果说明:
        仔细观察一下,就会发现,和left join的结果恰好相反,此次是以右表(B)为基础的,A表不足的地方用NULL填充.

3.inner join
sql语句以下:
select * from A
innerjoin B
on A.aID = B.bID

结果以下:aID               aNum                          bID                  bName1                   a20050111                1                      20060324012                   a20050112                2                     20060324023                   a20050113                3                     20060324034                   a20050114                4                     2006032404