function order($columns){
        $this->rows = null;
        if($columns != ""){
            foreach(func_get_args() as $columns){
                if($this->union){
                    $this->unionOrder[] = $columns;
                }else{
                    $this->order[] = $columns;
                }
            }
        }elseif($this->union){
            $this->unionOrder = array();
        }else{
            $this->order = array();
        }
        return $this;
    }