1.

 

select ... from A  left join B  on A.key = B.key

2.

select ... from A inner join B on A.key = B.key

3.

select ... from A right join B on A.key = B.key

4.

select ... from A left join B on A.key = B.key

where B.key is NULL

5.

 

select ... from A right join B on A.key = B.key

where A.key is NULL

6.

select ... from A outer join B on A.key = B.key

 

7. 这种情况需要注意！

select ... from A outer join B on A.key = B.key where  

A.key = NULL or B.key = NULL