MySQL七种join理论

select * from A inner join B where A.key=B.key;

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

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

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

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

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

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;