Du bör använda INTERSECT
och MINUS
(för 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')
Sedan UNION
lägger till undersökningar upp medan EXCEPT
/MINUS
subtraherar dem