AWS RDS MySQL - GROUP_CONCAT returns multiple rows instead of comma separated string

Question

I have a Stored Procedure that takes three parameters, one of which is TEXT and it should contain comma separated values with ids, something like this -> '12345,54321,11111,22222', and this it inserts a row with data for each id in the list. Below is the Stored Procedure:

DELIMITER //
-- Create Stored Procedure
CREATE PROCEDURE MyProcedure( 
        IN ItemUUID VARCHAR(255),
        IN ReceiverIds TEXT,
        IN ItemCreated VARCHAR(255)
)

BEGIN
  DECLARE strLen    INT DEFAULT 0;
  DECLARE SubStrLen INT DEFAULT 0;

  IF ReceiverIds IS NULL THEN
    SET ReceiverIds = '';
  END IF;

do_this:
  LOOP
    SET strLen = LENGTH(ReceiverIds);

    INSERT INTO item_receiver (item_uuid, receiver_id, item_created)
    VALUES (ItemUUID ,SUBSTRING_INDEX(ReceiverIds, ',', 1),ItemCreated);

    SET SubStrLen = LENGTH(SUBSTRING_INDEX(ReceiverIds, ',', 1)) + 2;
    SET ReceiverIds = MID(ReceiverIds, SubStrLen, strLen);

    IF ReceiverIds = '' THEN
      LEAVE do_this;
    END IF;
  END LOOP do_this;

END//
DELIMITER ;

To get comma separated values with ids, something like this -> '12345,54321,11111,22222' I execute subquery, however, when I call this Stored Procedure I get this error -> Error Code: 1242. Subquery returns more than 1 row

SET group_concat_max_len = 2048;
call MyProcedure('random_test_uuid',(
    SELECT CAST(GROUP_CONCAT(receiver_id SEPARATOR ',') AS CHAR)  AS receiver_ids FROM receiver
    WHERE user_id LIKE (SELECT user_id FROM user WHERE user_name LIKE 'myName') 
    GROUP BY receiver_id ),
'2017-09-24 23:44:32');

Answer 1

The problem is the subquery. Remove the group by:

SELECT CAST(GROUP_CONCAT(receiver_id SEPARATOR ',') AS CHAR)  AS receiver_ids
FROM receiver
WHERE user_id LIKE (SELECT user_id FROM user WHERE user_name LIKE 'myName') 

With the group by, you are getting a separate row for each receiver_id. The group_concat() is not doing anything.

Also, the CAST() is unnecessary. And this would typically be written as:

SELECT GROUP_CONCAT(r.receiver_id SEPARATOR ',') AS receiver_ids
FROM receiver r JOIN
     user u
     ON u.user_id = r.user_id
WHERE u.user_name LIKE 'myName';

If 'myName' is not using wildcards, then = is more appropriate than like.

If receiver_id is not unique in receiver, then you might want to add distinct to the group_concat().