public function groupBy($field)
    {
        // is the first time call groupBy method ?
        if ($this->_groupby_str == '') {
            $this->_groupby_str = ' GROUP BY '.self::_wrapRow($field);
        } else {
            $this->_groupby_str .= ' , '.self::_wrapRow($field);
        }

        return $this;
    }