SELECT с помощта на клауза UNION

Трябва да използвате INTERSECT и MINUS (за 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')

От UNION добавя подзаявки нагоре, докато EXCEPT /MINUS изважда тях