I guess your question boils down how you deal with optionality of fields. If all columns are required, a single table is more clean. If columns are optional, it

I guess your question boils down how you deal with optionality of fields. If all columns are required, a single table is more clean. If columns are optional, it makes sense to have a separate table for it. Furthermore, as you name it, once you want to extend your binary relations to triplets, for instance, the seperate table will pay off. However, a DB schema should be written for being optimal today and not in a nonexisting future.