Check if a row exists inside a MySQL function - mysql

Question

In my user table the IDs user_id are randomly generated by the following functions:
DELIMITER #
CREATE FUNCTION GenerateRandomUserID()
RETURNS CHAR(15)
BEGIN
DECLARE user_id CHAR(15) DEFAULT '';
WHILE LENGTH(user_id) < 15 DO
SET user_id = CONCAT(user_id, SUBSTRING('0123456789', RAND() * 10 + 1, 1));
END WHILE;
RETURN user_id;
END#
DELIMITER ;
DELIMITER #
CREATE FUNCTION GenerateUniqueUserID()
RETURNS CHAR(15)
BEGIN
DECLARE user_id CHAR(15) DEFAULT '';
REPEAT
SET user_id = GenerateRandomUserID();
UNTIL NOT UserIDExists(user_id) END REPEAT;
RETURN user_id;
END#
DELIMITER ;
I then insert a user generating its ID with this function. So far so good. The problem is when I insert another user, which generates an infinite loop. I managed to boil down the problem to yet another function, UserIDExists, which always returns TRUE (it shouldn't, obviously).
DELIMITER #
CREATE FUNCTION UserIDExists(user_id CHAR(15))
RETURNS BOOL
BEGIN
RETURN EXISTS(SELECT 1 FROM `user` WHERE `user_id` = user_id);
END#
DELIMITER ;
This last function does not do what it's supposed to, but I can't figure out why. If a single user exists with any password this will return TRUE, only if there are no users does it return FALSE. Can anyone figure out why?
Update:
I also tried this. Same result:
DELIMITER #
CREATE PROCEDURE UserIDExists(IN user_id CHAR(15), OUT user_id_exists BOOL)
BEGIN
SET user_id_exists = 0;
SELECT 1 INTO user_id_exists FROM `user` WHERE `user_id` = user_id;
END#
DELIMITER ;
Update:
Tried this as well:
DELIMITER #
CREATE PROCEDURE UserIDExists(IN user_id CHAR(15), OUT user_id_exists BOOL)
BEGIN
IF ((SELECT `user_id` FROM `user` WHERE `user_id` = user_id) != NULL) THEN
SET user_id_exists = TRUE;
ELSE
SET user_id_exists = FALSE;
END IF;
END#
DELIMITER ;
Update:
I tried a few variantes with the following:
DELIMITER #
CREATE PROCEDURE UserIDExists_2(IN user_id CHAR(15), OUT user_id_exists CHAR)
BEGIN
SELECT `user_id` FROM `user` WHERE `user_id` = user_id;
END#
DELIMITER ;
And came to the conclusion that this returns 1 when user_id is also 1 and returns nothing for any other value. This is pretty odd, because this is the only row I have on the table:
# user_id, first_name, last_name, pwd, is_confirmed
'252316605573186', 'André', 'Fratelli', NULL, '0'
This is getting on my nerves...

Answer 1

This works. I though that the quotes where enough to distinguish the procedure's arguments from the table's columns, but I guess I was wrong.
DELIMITER #
CREATE PROCEDURE UserIDExists(IN p_user_id CHAR(15), OUT p_user_id_exists BOOL)
BEGIN
SET p_user_id_exists = EXISTS(SELECT `user_id` FROM `user` WHERE `user_id` = p_user_id);
END#
DELIMITER ;