Codd himself, some years after publication of his 1970 model, proposed a [[three-valued logic]] (True, False, Missing or /[[Null (SQL) |NULL]]) version of it to deal with missing information, and in his ''The Relational Model for Database Management Version 2'' (1990) he went a step further with a four-valued logic (True, False, Missing but Applicable, Missing but Inapplicable) version.<ref>{{cite book| first =C.J | last = Date|title=Date on Database: Writings 2000–2006|date= 2006 |publisher= Apress|isbn= 978-1-59059-746-0|pages=329–41|chapter= 18. Why Three- and Four-Valued Logic Don't Work}}</ref> But these have never been implemented, presumably because of attending complexity. SQL's NULL construct was intended to be part of a three-valued logic system, but fell short of that due to logical errors in the standard and in its implementations.<ref>{{cite book | first =C.J | last = Date|title=An Introduction to Database Systems|date=2004|publisher= Addison Wesley |isbn=0-321-19784-4|pages=592–97|edition= 8}}</ref>
