public function order()
    {
        $columns = func_get_args();
        $order = '';
        $i = 0;
        //
        if ($columns) {
            foreach ($columns as $value) {
                if (!$i) {
                    $order .= ' order by '.$value;
                } else {
                    $order .= ','.$value;
                }
                $i = 1;
            }
        }
        //
        $this->order = $order;
        //
        return $this;
    }