Using IF-ELSE OR CASE-WHEN in Magento Collection - Expression based field by addExpressionAttributeToSelect

Using IF-ELSE OR CASE-WHEN  in Magento Collection - Expression based temporary column field  by addExpressionAttributeToSelect

Magento addExpressionAttributeToSelect  method can be used to create temporary column in
collections

$_collection = Mage::getResourceModel('sales/order_collection')
        ->addExpressionAttributeToSelect('your_temp_column', 'CASE order_id WHEN 1 THEN one ELSE TWO END ', '');

on printing sql query using getselect
echo $_collection->getSelect() ;


 the magento will create query something like this  SELECT (CASE order_id WHEN 1 THEN one ELSE TWO END) AS your_temp_column

The temproary column can be used for sorting and filtering also , please note for filtering
you need to use 'having'  instead of builtin addAttributeToFilter method.
$_collection->getSelect()->having('your_temp_column = "one" '); 

hope this helps