public function getConnection()
{
    if (!$this->_connection) {
        $this->_setConnection();
    }
    return $this->_connection;
}

private function _setConnection()
{
    if (!$this->_connection) {
        $this->_connection = Mage::getSingleton('core/resource')->createConnection(self::EXTERNAL_RESOURCE_NAME, 'pdo_mysql', $this->getConfig());
    }
}

$dbConnection = $this->getConnection();
$sql = $dbConnection->query("select...");
$res = $sql->fetchAll();