子表有记录无法删除用触发器

CREATE OR REPLACE TRIGGER SM5606btnDelete_TR
BEFORE DELETE ON SM_cg_CGFP1_TB
FOR EACH ROW
DECLARE
V_COUNT NUMBER;
BEGIN
CASE
WHEN DELETING THEN
SELECT COUNT(1)
INTO V_COUNT
FROM SM_cg_CGFP2_TB
WHERE FID=:OLD.ID;
END CASE;
IF V_COUNT > 0 THEN
RAISE_APPLICATION_ERROR(-20002, :OLD.FPBH||'存在明细,不可删除请先删除明细');
END IF;
END;