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> Maybe we should have introduced NEX (non-existing) and NAP (non-applicable) as possible values in addition to NULL?

Codd (the inventor of relational algebra) actually suggested this. I think the primary source is a book that may not be on the web. There's some discussion here (mostly saying why they think it didn't happen and wouldn't work out): https://arxiv.org/html/1606.00740v1/




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