public function select()
    {
        $cols = func_get_args();

        if ( ! func_num_args() || in_array('*', $cols)) {
            $this->_cols_str = ' * ';
        } else {
            // _cols_str default ' * ' , it easy to get a result when select func dosen't called
            // but when you call select func , you should set it to ''
            $this->_cols_str = '';
            foreach ($cols as $col) {
                $this->_cols_str .= ' '.self::_wrapRow($col).',';
            }
            $this->_cols_str = rtrim($this->_cols_str, ',');
        }

        return $this;
    }