best way to to a "union" in a table?

I have a data structure that I need to represent in a table.

It is a changelist, and the contents of the changelist are either

• an existing element in the system, in which case it already has a unique identifier, or a
• new element to be added to the system, in which case it does not have an assigned unique identifier, but instead a string identifying a disk file path.

What's the best way to represent this as an sql table? The obvious way is something like:

changelist_number NUMBER
entry_type -- either MODIFY or NEW identifier NUMBER -- if entry_type is MODIFY filepath VARCHAR -- if entry_type is ADD

Am I going down the right path?

Many TIA!
Mark

-- 
Mark Harrison
Pixar Animation Studios