create
    definer = `mysql.sys`@localhost function sys.list_add(in_list text, in_add_value text) returns text comment '
Description
-----------

Takes a list, and a value to add to the list, and returns the resulting list.

Useful for altering certain session variables, like sql_mode or optimizer_switch for instance.

Parameters
-----------

in_list (TEXT):
  The comma separated list to add a value to

in_add_value (TEXT):
  The value to add to the input list

Returns
-----------

TEXT

Example
--------

mysql> select @@sql_mode;
+-----------------------------------------------------------------------------------+
| @@sql_mode                                                                        |
+-----------------------------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+-----------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> set sql_mode = sys.list_add(@@sql_mode, ''ANSI_QUOTES'');
Query OK, 0 rows affected (0.06 sec)

mysql> select @@sql_mode;
+-----------------------------------------------------------------------------------------------+
| @@sql_mode                                                                                    |
+-----------------------------------------------------------------------------------------------+
| ANSI_QUOTES,ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+-----------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

' deterministic sql security invoker
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 the new value as a single value list
        RETURN in_add_value;
    END IF;
    RETURN (SELECT CONCAT(TRIM(BOTH ',' FROM TRIM(in_list)), ',', in_add_value));
END;