$orders = Mage::getModel('sales/order')->getCollection();
$orders->getSelect()->reset('columns');
$orders->getSelect()->columns(array('total_amount'=>'sum(grand_total)'));
$orders = Mage::getModel('sales/order')->getCollection();
$orders->getSelect()->reset('columns');
$orders->getSelect()->columns(array('total_amount'=>'sum(grand_total)'));