select * from mycduser u, mycdprod p
where u.userid(+) = p.userid;
방법1.
SELECT DISTINCT P.USERID
FROM PROD_INF P LEFT OUTER JOIN USER_MST U ON U.USERID = P.USERID
WHERE U.USERID IS NULL;
방법2.
SELECT DISTINCT USERID
FROM PROD_INF A
WHERE 1 = 1
AND NOT EXISTS(SELECT 'X'
FROM USER_MST B
WHERE A.USERID = B.USERID);