sort on fields that are not mandatory in each document

[Derek Poh]

Hi

I am trying to sort on multiple fields. These fields donot necessary 
exist in every document.
sort=sppddrank asc, ddrank asc
From the sorted result, it seems that documents which donot have 
sppddrank field is at the top.

How can I make the documents that have the sppddrank field to be on top 
and sortedby it and those documents which do not have the field below?

-Derek