--內連接(inner) 可以省略inner
select a_UserID,b_StudentID from Table_a
inner join Table_b on a_UserID = b_StudentID 

--外連接之左連接 也可以寫成left join（省略outer）
select a_UserID,b_StudentID from Table_a
left outer join Table_b on a_UserID = b_StudentID 

--全外連接
select a_UserID , b_StudentID from Table_a
full outer join Table_b on a_UserID=b_StudentID

--自身連接
select s.a_ID , s.a_UserName, i.a_UserID, i.a_UserName from Table_a i
join Table_a s on i.a_UserID = s.a_ID

--交叉連接
select i.a_UserID,t.b_StudentID from Table_a i cross join Table_b t
select count(*) from Table_a i cross join Table_b t