Thắc mắc về SQL

[Anh_Minh1 (beminah1999)]

DROP TRIGGER IF EXISTS khuyen_mai;
DELIMITER $$
CREATE TRIGGER khuyen_mai BEFORE UPDATE ON payment FOR EACH ROW
BEGIN
	DECLARE finished_1 INTEGER DEFAULT 0;
    DECLARE finished_2 INTEGER DEFAULT 0;
    DECLARE c_id INT DEFAULT 0;
    DECLARE am DOUBLE;
	DECLARE cs CURSOR FOR 
    	SELECT customer_id,SUM(amount) FROM sakila.payment GROUP BY customer_id HAVING SUM(amount) > 50;
	DECLARE CONTINUE HANDLER 
        FOR NOT FOUND SET finished_1 = 1;
	OPEN cs;
    WHILE finished_1 = 0 DO
	FETCH cs INTO c_id,am;
    if(new.customer_id = c_id AND am < 100) then
    INSERT INTO khuyen_mai(rental_id,amount) VALUES (new.rental_id, (new.amount*9)/10);
    elseif(new.customer_id = c_id AND am > 100) then
    INSERT INTO khuyen_mai(rental_id,amount) VALUES (new.rental_id, (new.amount*8)/10);
    end if;
    END WHILE;
    CLOSE cs;
	
END$$
DELIMITER ;