Take a look at http://magento.stackexchange.com/questions/19863/how-to-convert-sql-query-to-magento-in-order-to-fetch-data

    $read = Mage::getSingleton('core/resource')->getConnection('core_read');
    
    $readresult = $read->query("SELECT id FROM customers_old");
    
    while ($row = $readresult->fetch() ) {
        $Ids[] = $row['id'];
    }