recursive mysql procedure for getting a referral branch - mysql

Question

I recently got acquainted with procedures and functions in mysql and still don't quite understand how they work. I'm trying to make a referral branch to display on the site.
DELIMITER $$
CREATE PROCEDURE fb2_get_referrals (IN refer_id INT UNSIGNED, IN parent_level INT UNSIGNED, OUT return_path MEDIUMTEXT)
BEGIN
DECLARE parent_id INT UNSIGNED;
DECLARE path_result MEDIUMTEXT;
DECLARE referral_offset INT UNSIGNED;
DECLARE count_user_referrals INT UNSIGNED;
SET referral_offset = 0;
SET parent_level = parent_level + 1;
SET count_user_referrals = (SELECT COUNT(fb2_users.id) FROM fb2_users WHERE fb2_users.referral_id = refer_id);
IF(count_user_referrals > 0) THEN
label1: WHILE referral_offset < count_user_referrals DO
SELECT
CONCAT(
'{',
'"id":',fb2_users.id,',',
'"level":',parent_level,
'}'
) as path,
fb2_users.id as pi,
parent_level as pl
INTO return_path, parent_id, parent_level
FROM fb2_users
WHERE fb2_users.referral_id = refer_id
LIMIT 1 OFFSET referral_offset;
CALL fb2_get_referrals(parent_id, parent_level, path_result);
SELECT CONCAT(path_result, return_path) INTO return_path;
SET referral_offset = referral_offset + 1;
END WHILE label1;
END IF;
END$$
DELIMITER $$
CREATE FUNCTION getPath(user_id INT) RETURNS MEDIUMTEXT DETERMINISTIC
BEGIN
DECLARE res MEDIUMTEXT;
CALL fb2_get_referrals(user_id,0, res);
RETURN res;
END$$
id
r_id
1
null
2
null
3
null
4
null
5
null
6
1
7
6
8
6
9
7
as a result, I get null