在mysql中使用规则异常的If语句？

IF search_condition THEN statement_list
[ELSEIF search_condition THEN statement_list] ...
[ELSE statement_list]
END IF;

DELIMITER //
CREATE PROCEDURE CheckAge(IN p_age INT)
BEGIN
    IF p_age < 18 THEN
        SELECT 'Minor';
    ELSEIF p_age BETWEEN 18 AND 65 THEN
        SELECT 'Adult';
    ELSE
        SELECT 'Senior';
    END IF;
END //
DELIMITER ;

DELIMITER //
CREATE PROCEDURE CalculateDiscount(IN p_price DECIMAL(10, 2), IN p_discount_rate DECIMAL(5, 2))
BEGIN
    DECLARE final_price DECIMAL(10, 2);
    IF p_discount_rate < 0 THEN
        SET final_price = p_price;
    ELSEIF p_discount_rate > 1 THEN
        SET final_price = p_price * 0.9; -- 默认折扣为10%
    ELSE
        SET final_price = p_price * (1 - p_discount_rate);
    END IF;
    SELECT final_price;
END //
DELIMITER ;