Sharepoint 2010 Exclusive columns?

Does anyone know a way to have two columns in a SP2010 list that are exclusive? I need to ensure that there is only a value for ONE column, not both.

Basically I need the following structure:

Category List ->  
SubCategory List (with a lookup to Category) ->  
Value (with a lookup to SubCategory).  

But, if there is no SubCategory, use a lookup to Category. One 开发者_如何学Pythonor the other must be used, not both.

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Using either a list Event Receiver (SPItemEventReceiver) or a Custom Workflow should be able to achieve these desired semantics.

I do not believe the model itself can represent such relationships. An alternative might be to have different content types, for which only one of the columns applies (to each).

Happy coding.