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Yous have another column which is a flag giving the stae of the attribute. Note that not known and not valid for the row are two different states. If you set the attribute to NULL - which of the two do you mean?

Actually you give the answer for attribute not applicable - and you don't get a thousand different subcases. The measure of goodness is does your db model fit the real world data.

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So I end up having values in the actual column that may be complete junk and I need to check another column to know that? I would expect that to cause trouble sooner or later.

Right. Attempting that workflow would be crazy without language support.

But at the same time, proper languages that have operator overriding and custom types make that kind of thing trivial - look at how many languages have a Maybe<T> these days, which is basically automating that process.

The fact that doing it in SQL would be suicidal shows that, while SQL was a brilliant language for the '70s... it has not evolved nearly enough.

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