Compare 3 tables in SQL, with result found in A, B, not C

Question

I am trying to get results from 3 tables, but to show records that reflect Users from A also in B, but not in C

Table A   =   Table B  not in   Table C
-------      --------          --------
UserName     UserName           UserName

I apologize if this is too basic. I'm new at this and could not come up with anything from things I googled.

Answer 1

There are a couple ways you could do this, one would be by using a LEFT JOIN:

Select      *
From        TableA  A
Join        TableB  B   On  A.UserName = B.UserName
Left Join   TableC  C   On  A.UserName = C.UserName
Where       C.UserName Is Null;

Another would be by using a WHERE NOT EXISTS:

Select      *
From        TableA  A
Join        TableB  B   On  A.UserName = B.UserName
Where Not Exists
(
    Select  *
    From    TableC  C
    Where   C.UserName = A.UserName
)