SQL Division using 'not exists' in mysql

Question

I have the following table:

\'committee\' table

commname    profname
========================
commA       bill
commA       jack
commA       piper
commB       bi         


        

Answer 1

Your innermost select isn't using anything from itself in its where clause, so it's always finding something for piper. Try

select distinct b.profname from committee b
where not exists (
    select commname from committee a
    where a.profname = 'piper' and not exists  (
        select commname from committee c
        where c.profname=b.profname and c.commname=a.commname
    )
);