Koen Verbeeck (1/14/2014)
Option A doesn't seem directly to be a good idea, as this requires two surrogates keys in the fact table. However, only one of those two can be filled in at the time. This can lead to issues against the grain of the fact table.
I take your point Koen and your knowledge in this area is better than mine (:-)), but as long as there are surrogate keys that define 'Not Applicable' or 'Has Happened Yet' etc., then this isn't a problem is it? I agree that it would not be good to have NULL values.
For example, in the Kimball Data Warehouse Toolkit book, they cover the concept of a Promotions dimension in relation to Retail Sales; there is not always going to be a promotion in place and so the Sales fact table would include whatever surrogate key denotes 'no promotion in place', but it doesn't affect the grain.
Regards
Lempster