public function group($group){
        if(is_array($group)){
            $this->group= 'GROUP BY ';
            foreach ($group as $value) {
                $this->group .= $value.',';
            }
            $this->group= rtrim($this->group, ',');
            $this->group.=' ';
        }else{
            $this->group= 'GROUP BY '.$group.' ';
        }
    }