BEGIN  IF (in_add_value IS NULL) THEN SIGNAL SQLSTATE '02200' SET MESSAGE_TEXT = 'Function sys.list_add: in_add_value input variable should not be NULL', MYSQL_ERRNO = 1138; END IF;  IF (in_list IS NULL OR LENGTH(in_list) = 0) THEN RETURN in_add_value; END IF;  RETURN (SELECT CONCAT(TRIM(BOTH ',' FROM TRIM(in_list)), ',', in_add_value));  END