DELIMITER $$
CREATE PROCEDURE getPaymentPeriod(IN studId INT, IN groupId INT, IN paymentYear INT)
BEGIN
DECLARE countOfMonths int;
DECLARE monthStr VARCHAR(10);
DECLARE yearStr VARCHAR(10);
SET monthStr = 'MONTH';
SET yearStr = 'YEAR';
    SELECT COUNT(*)
    INTO countOfMonths
    FROM taxespayment
    WHERE student_id = studId
    AND group_id = groupId
    AND year = paymentYear;

CASE countOfMonths
    WHEN 0 THEN SELECT 'This student has not paid for this group/year!' as PAYMENT_PERIOD;
    WHEN 1 THEN SELECT CONCAT('ONE_',monthStr) as PAYMENT_PERIOD;
    WHEN 3 THEN SELECT CONCAT('THREE_',monthStr,'S') as PAYMENT_PERIOD;
    WHEN 6 THEN SELECT CONCAT('SIX_',monthStr,'S') as PAYMENT_PERIOD;
    WHEN 12 THEN SELECT yearStr as PAYMENT_PERIOD;
    ELSE
            SELECT CONCAT(countOfMonths, monthStr, 'S') as PAYMENT_PERIOD;
        END CASE;
END$$;
DELIMITER ;