SQl 的左，右，内，全连接

select * from Student
select * from ClassForm

select [Name],ClassName from Student as a left join ClassForm as b on  a.ClassID=b.ID

select [Name],ClassName from Student as a right join ClassForm as b on  a.ClassID=b.ID

select [Name],ClassName from Student as a inner join ClassForm as b on  a.ClassID=b.ID

select [Name],ClassName from Student as a full join ClassForm as b on  a.ClassID=b.ID