[fw-db] How to select sum(amount) by Zend FrameWork

I did, but it shown:

Code:
Uncaught exception 'Zend_Db_Table_Row_Exception' with message 'Specified
column "SUM(amount)" is not in the row' in
for

PHP Code:
$objNews = new Salary();

$objNews->select()->from('Salary',array('SUM(amount)'))->where("name =
'john'");
$objNewsArray = $objNews->fetchAll ();
foreach($objNewsArray as $objNewsvar)
{
echo $objNewsvar["SUM(amount)"];
}
Class Salary extend from Zend_Db_Table_Abstract

I've tried array('SUM(amount)') or array('thecolumn'=>'SUM(amount)'), but it
not working properly.

Please help me.

Thanks,
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