SELECT pomocí klauzule UNION

Měli byste použít INTERSECT a MINUS (pro 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')

Od UNION přidá poddotazy nahoru, zatímco EXCEPT /MINUS odečte je