CREATE TABLE inv(itemno INT, stock INT);
CREATE TABLE sales(itemno INT, sold INT);
MERGE INTO inv AS T 
 USING (SELECT itemno, sum(sold) AS sold
          FROM sales
          GROUP BY itemno) AS S
 ON T.itemno = S.itemno
 WHEN MATCHED AND stock > sold
  THEN UPDATE SET stock = stock - sold
 WHEN MATCHED AND stock = sold THEN DELETE
 WHEN MATCHED THEN SIGNAL badstock  
 WHEN NOT MATCHED AND sold <  0 
  THEN INSERT VALUES(itemno, -sold)
 WHEN NOT MATCHED THEN SIGNAL badstock;