protected function quoteIdentifier($str)
    {
        $open = '"';
        $close = '"';
        switch ($this->pdoDriver) {
            case 'mysql':
                $open = '`';
                $close = '`';
                break;
            case 'mssql':
                $open = '[';
                $close = ']';
                break;
        }
        return $open . str_replace($open, $open . $open, $str) . $close;
    }