SQL求差：在table1但不在table2中

SELECT *
FROM (SELECT DISTINCT yy, mm, Org_id, bus_id
        FROM Monthorgsell) t1
WHERE (NOT EXISTS
          (SELECT DISTINCT yy, mm, Org_id, bus_id
         FROM MonthorgPlan AS t2
         WHERE t1.yy = t2.yy AND t1.mm = t2.mm AND t1.org_id = t2.org_id AND
               t1.bus_id = t2.bus_id))