[tpmoney]

But the "known absent" value is going to be different for different domains. For example, in EEOC databases the "known absent" value for a race would be "declined to answer". In a database of test scores, it might be "Didn't complete", but it could also be "was absent from class on exam day" so SQL can't specify what that is. On the other hand "this value is unknown" can use the same marker in all domains, and SQL chose NULL as that marker. To be completely strict about it, "have a value/don't have a value" is one piece of data if that's something you care about and "what is that value" is another one. So in an ideal system, you should have a column for "value is [present | known absent | unknown]" and a separate column for the actual value when "value is present"

Most of the time it's not that important and people can and do shortcut "null" to mean "not present" but then the issues with using null in equality statements is a result of taking the short cut, not necessarily with the logic around null.