I'd like to add that, in the above example, you get bitten by NULL semantics even though you don't have any NULLs in your data at all. So if you don't start ou

I'd like to add that, in the above example, you get bitten by NULL semantics even though you don't have any NULLs in your data at all. So if you don't start out with any NULLs, and you have complete and unambiguous knowledge of your entire data set, how does "unknown" get injected into such an innocent-looking example?