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> For context, NULL/missing was a type. N/A was a type (i.e. a question irrelevant to someone). I can't remember the 3rd, and of course not the additional (4th or 5th) type he was proposing

“Don’t know”: this has a value, but we do not know it could be one. Also, within N/A, one _could_ discriminate between “not available yet”, “will never be available”, “was available once, but was lost”, etc.

Depending on the domain, others could be

- “Can’t tell”: this has a value, but you are not allowed not know it (unlikely, as you likely also shouldn’t be allowed that the value exists)

- incomputable: this has a value, but it isn’t possible to know it

I think generic systems shouldn’t try to capture such domain specific things, but allow for implementing them. SQL shouldn’t even have null, but have enums and product types on top of which you could create it and functions supporting it.

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This reminds me of the sixteen or so different flavors of null used in HL7! https://terminology.hl7.org/en/CodeSystem-v3-NullFlavor.html though in practice I've only seen a handful of these. Notably:

NI No Information - the default, it is unknown and why it is unknown is also unknown

UNK Unknown - the most common other than NI

ASKU Asked but Unknown

NASK Not Asked - we don't know because we didn't ask

MSK Masked - hidden due to privacy or legal etc

NA Not Applicable

NAV Not Available - for example patient might be unconscious or doctor might be unreachable, temporary

"Don't care" - it has a value, but it doesn't matter. This is classically used in Karnaugh maps.

But you can imagine two kinds of "don't care" values, too: the complacent one, and the assertive one. The complacent one is the one we have in Karnaugh maps, which says, "it doesn't matter what I am, so if it matters to you what I am, I'll be what you want". But the assertive variant says, "I don't matter, and if I matter for your calculation, your calculation clearly doesn't matter either."

How is “not available yet” different from "Don't know"? How is incomputable different from “will never be available”?

> SQL shouldn’t even have null

But it needs to, because it needs to represent half of a result record not existing, i.e. on a LEFT JOIN.

"Not available yet", at least to me, carries the implication that it will, not just might, become available at some point. Probably soon enough that it's worth asking the question again later. Whereas "don't know" suggests that it's not worth asking the question again, because you're probably just going to get "don't know" as the answer. (At least, if "don't know" and "not available yet" are both presented as options).

A more accurate description is “May become available”, it might be possible that it could become available and we believe it could within a short order, however it is also not assured or guaranteed that it will become available when we think, and it may also never become available at all for any number of reasons as simple as someone not following up and adding newly ascertained information or that the only possible source of the information has become permanently unavailable for many other reasons of their own.

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