-- ------------------------------------------------------
-- Example: What Customers Do NOT have shipping addresses
-- ------------------------------------------------------
select custid from customer
  where custid not in (select custid from soaddress)

-- -------------------------------------------------------
-- Example: What Projects exist in PjProj and not pjprojem
-- -------------------------------------------------------
select project from pjproj
  where project not in (select project from pjprojem)

-- Alternatively...
select project from pjproj
  where not (project in (select project from pjprojem))