If you look at the code attempted by Murali, he is trying to check for null and his description mentioned source file so depending on a flat file or table source how you are going to check for the value is going to change.

I am not sure if you are a script only programmer like one my colleague who will always insist on scripts even if multiple options are available.

I am not an expert to comment if script or derived column function is the best solution but for this purpose derived column will be lot simple and easy to code. That is just my opinion.

Peter