public function getPart($part)
    {
        $part = strtolower($part);
        if (!array_key_exists($part, $this->_parts)) {
            // require_once 'Zend/Db/Select/Exception.php';
            throw new Zend_Db_Select_Exception("Invalid Select part '$part'");
        }
        return $this->_parts[$part];
    }