I thought the issue was zero vs. null. If it was just in the context of a FK, my averaging example was not applicable. Null is still the correct value for the FK.

>What it has to do with zero in FK?
>
>>You don't? I am not badgering you but that is not the kind of thing I would expect a SQL expert (which you are) to say. Zero is a value. Null means unknown. One area where the distinction is crucial is in group computations. Let's say you want average sales last month for all the salespeople. If one of them sold zero, that should be part of the computation. If for whatever reason one of them has a null value for sales, that should not be included.
>>
>>Maybe there is something I am missing. I have a good understanding of relational databases as conventionally taught but do not claim to be an expert. There are always things I can learn.