问Sql触发器“列列表对此触发器类型无效”

EN

CREATE TRIGGER add_primary_key
AFTER INSERT ON CAJ
FOR EACH ROW
WHEN (ID_CAJ IS NULL)
DECLARE
maximum NUMBER;
BEGIN
  maximum := SELECT MAX(ID_CAJ) FROM caj;
  if (maximum <> 0)
    UPDATE caj
    SET ID_CAJ = maximum + 1;
    WHERE CAJ_ID = NULL
    maximum = maximum +1;
END;

"invalid NEW or OLD specification"
*Cause:    An invalid NEW or OLD specification was given for a column.
*Action:   Re-specify the column using the correct NEW or OLD specification.