SQL 连接(JOIN) https://www.runoob.com/sql/sql-join.html
select *from Users u left join Classes c on u.id=c.userid
select *from Users u right join Classes c on u.id=c.userid
select *from Users u join Classes c on u.id=c.userid
select *from Users u left join Classes c on u.id=c.userid where c.userid is null
select *from Users u right join Classes c on u.id=c.userid where u.id is null
select *from Users u full outer join Classes c on u.id=c.userid
select *from Users u full outer join Classes c on u.id=c.userid where c.userid is null or u.id is null