如何强制两个外键列都为null或都指向Oracle中其他表中的某一行？

CREATE OR REPLACE TRIGGER check_foreign_keys
BEFORE INSERT OR UPDATE ON your_table
FOR EACH ROW
DECLARE
    v_count NUMBER;
BEGIN
    -- 检查外键列1的值
    SELECT COUNT(*) INTO v_count
    FROM table1
    WHERE table1_id = :NEW.foreign_key1;
    
    IF v_count = 0 THEN
        RAISE_APPLICATION_ERROR(-20001, '外键列1的值无效');
    END IF;
    
    -- 检查外键列2的值
    SELECT COUNT(*) INTO v_count
    FROM table2
    WHERE table2_id = :NEW.foreign_key2;
    
    IF v_count = 0 THEN
        RAISE_APPLICATION_ERROR(-20002, '外键列2的值无效');
    END IF;
END;
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