VÆLG ved hjælp af UNION-klausul

Du skal bruge INTERSECT og MINUS (for Oracle):

-- At least at Boston or at Dallas 
(SELECT DNAME 
   FROM DEPARTMENT 
  WHERE CITY = 'DALLAS'

UNION

 SELECT DNAME 
   FROM DEPARTMENT 
  WHERE CITY = 'BOSTON')

MINUS

-- At Boston and at Dallas
(SELECT DNAME 
   FROM DEPARTMENT 
  WHERE CITY = 'DALLAS'

INTERSECT

 SELECT DNAME 
   FROM DEPARTMENT 
  WHERE CITY = 'BOSTON')

Siden UNION tilføjer underforespørgsler op, mens EXCEPT /MINUS fratrækker dem