在这样的表R(ABCDE)上，我需要一个SQL触发器来防止元组的A和B列相同，但尝试为C赋予不同的值

CREATE TRIGGER check_duplicate
BEFORE INSERT ON R
FOR EACH ROW
BEGIN
    IF NEW.A = NEW.B THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'A and B values cannot be the same';
    END IF;
    IF NEW.A = OLD.A AND NEW.B = OLD.B THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'A and B values cannot be updated to the same values';
    END IF;
    IF NEW.A = OLD.A AND NEW.C = OLD.C THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'A and C values cannot be the same';
    END IF;
END;