How to get sum of particular column (custom model)?

Question

I have custom model with 4 fields(a,b,c,d). I need sum of "d" field. So my solution using collection loop the d values and make the sum.For this is any simple solution? Thanks in advance.

  $available=0;
    $pieces = Mage::getResourceModel('rad_inventorypieces/piece_collection')
    foreach($pieces as piece)
    {

$available=+$piece->getAvailable();
    }

Answer 1

It is better to implement new method in class of rad_inventorypieces/piece resource model

Method might look like this:

/**
 * @return string
 */
public function getSumD()
{
    $select = $this->getReadConnection()
        ->select()
        ->from($this->getMainTable(), array('sum' => new Zend_Db_Expr('SUM(d)')));

    return $this->getReadConnection()->fetchOne($select)
}