MySQL七种JOIN方式

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

select A.* from A left join B on A.key=B.key where B.key IS NULL
UNION
select B.* from A right join B on A.key=B.key where A.key IS NULL

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

select A.*,B.* form A left join B on A.key=B.key;

select A.* from A left join B on A.key=B.key where B.key IS NULL

select A.*,B.* form A right join B on A.key=B.key;

select B.* form A right join B on A.key=B.key where A.key IS NULL;