CREATE OR REPLACE TRIGGER my_trg BEFORE INSERT OR UPDATE ON Relationships FOR EACH ROW
    DECLARE FUNCTION i_Age
    (id NUMBER) RETURN NUMBER IS
    li_res NUMBER;
    BEGIN
        SELECT p.AGE INTO li_res FROM PERSONS p WHERE p.ID = id;
        RETURN li_res;
    END;

    BEGIN
        IF INSERTING OR UPDATING THEN
            IF :NEW.RELATION = 'child' AND i_Age(:NEW.PERSON_ID) >= i_Age(:NEW.RELATIVE_ID) THEN
                RAISE INVALID_NUMBER;
            END IF;
        END IF;
    END;