A.SELECT r1 AS retval FROM tab1 INTERSECT SELECT r2 AS retval FROM tab2
B.SELECT r1 AS retval FROM tab1 EXCEPT SELECT r2 AS retval FROM tab2
C.SELECT DISTINCT r1 AS retval FROM tab1, tab2 WHERE r1 r2
D.SELECT r1 AS retval FROM tab1 UNION SELECT r2 AS retval FROM tab2
参考答案：B
