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