> 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)

> 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/ https://arxiv.org/html/1606.00740v1/