SQL 左连接，右连接，全连接

A  表               B 表   
----------        -----------------
c  列                d列
1                    4
2                    5
3                    6
4                    7
5



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


1.全连接
select A.c,B.d from A   join B on A.c=B.d;


c   d
4   4
5   5


2.左连接
select A.c,B.d from A  left join B on A.c=B.d;


c   d
1
2
3
4   4
5   5
  

3.右连接
select A.c,B.d from A  right join B on A.c=B.d;


c    d
4    4
5    5
     6
     7