    public static function run($query, $database = null)
    {
        if (empty(self::$connect)) {
            self::connect();
        }
        if ($database != self::$config['database']) {
            self::$connect->select_db($database);
        }
        if (!self::$connect->query($query)) {
            system::add_error('sql::run()', 'query_fail', 'fail to run query');
        }
    }