Answer by Stephen S. for Nullable Foreign Key bad practice?

You could always add an artificial row to your Customer table, something like Id=-1 and CustomerName = 'Unknown' and then in cases when you would normally set your CustomerId in Order NULL set it to -1.

This allows you to have no nullable FKs but still represent the lack of data appropriately (and will save you from downstream users not knowing how to deal with NULLs).