It does not work collection queries with the condition - mysql

Question

I need to bring to the customer a discount depending on how long it took a training course.
To find out the date of passage must first know the group ID to which the client belongs.
When I know Id of group, I can find date when started training course.
And then, determined the size of discounts by finded date
Here's a script that works great in MS SQL, but in MySql it doesn't works.
CREATE DEFINER=`root`#`localhost` PROCEDURE `GetBonus`(
idStudent CHAR(36)
)
BEGIN
IF (SELECT COUNT(*) FROM (SELECT IdGroupe FROM GroupeStudent WHERE IdStudent = idStudent) groupeCourse) = 1 THEN
BEGIN
DECLARE CourseGroupeDate datetime;
SET CourseGroupeDate = (SELECT Date FROM CourseGroupe WHERE Id = (SELECT IdGroupe FROM GroupeStudent WHERE IdStudent = idStudent));
IF (CourseGroupeDate - INTERVAL NOW() MONTH) < 3 AND
(CourseGroupeDate - INTERVAL NOW() MONTH) > 0
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000000';
ELSEIF (CourseGroupeDate - INTERVAL NOW() MONTH) < 6 AND
(CourseGroupeDate - INTERVAL NOW() MONTH) > 3
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000001';
ELSEIF (CourseGroupeDate - INTERVAL NOW() MONTH) < 12 AND
(CourseGroupeDate - INTERVAL NOW() MONTH) > 6
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000002';
END IF;
END;
END IF;
END

Answer 1

CREATE PROCEDURE `GetBonus`(
idStudent CHAR(36)
)
BEGIN
IF (SELECT COUNT(groupeCourse.IdGroupe) FROM (SELECT IdGroupe FROM GroupeStudent WHERE GroupeStudent.IdStudent = idStudent) groupeCourse) = 1 THEN
BEGIN
SET #CourseGroupeDate := (SELECT Date FROM CourseGroupe WHERE Id = (SELECT GroupeStudent.IdGroupe FROM GroupeStudent WHERE GroupeStudent.IdStudent = idStudent));
IF (TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) < 3 AND
(TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) > 0
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000000';
ELSEIF (TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) < 6 AND
(TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) > 3
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000001';
ELSEIF (TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) < 12 AND
(TIMESTAMPDIFF(MONTH, #CourseGroupeDate, now())) > 6
THEN SELECT * FROM Discounts WHERE Id = '00000000-0000-0000-0000-000000000002';
END IF;
END;
END IF;
END
All in the details , for example , it was necessary to specify the table name about the field GroupeStudent.IdStudent = idStudent