public static function extractColumns() 
    {
        self::_checkLoaded();
        $cls = get_called_class();
        $temp_columns = "";
        $escape = Driver::$escape_char;
        foreach (self::$_columns[$cls] as $column) {
            $temp_columns .= "$escape".self::getTableName()."$escape.$escape".self::$_mapping[$cls][$column]."$escape,";
        }
        return substr($temp_columns, 0, strlen($temp_columns)-1);
    }