mysql 7 种 join

一、

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

二、

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

三、

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

四、

select * from A left join B on A.key = B.key where b.key is null

五、

select * from A right join B on A.key = B.key where A.key is null

六、

select * from A left join B on A.key = B.key
union
select * from A right join B on A.key = B.key

七、

select * from A left join B on A.key = B.key where B.key is null
union
select * from A right join B on A.key = B.key where A.key is null