Magento Select Query

$connectionWrite = Mage::getSingleton('core/resource')->getConnection('core_read');
$resource = Mage::getSingleton('core/resource');
$tableName = $resource->getTableName('marketplace_product');
$query = 'SELECT userid FROM ' . $tableName . ' WHERE mageproductid = '.(int)$product_Id ;

$userid = $connectionWrite->fetchOne($query);