---

DELIMITER //
CREATE PROCEDURE AddZadacha (IN num INT, OUT result VARCHAR(100))
BEGIN
    IF num % 10 = 1 AND num % 100 != 11 THEN
        SET result = CONCAT(num, ' задача');
    ELSEIF num % 10 IN (2,3,4) AND num % 100 NOT IN (12,13,14) THEN
        SET result = CONCAT(num, ' задачи');
    ELSE
        SET result = CONCAT(num, ' задач');
    END IF;
END //
DELIMITER ;

-- Пример
CALL AddZadacha(21, @res);
SELECT @res;  -- 21 задача