Editing Relational model (section)

=== Extensions ===
Some years after publication of his 1970 model, Codd proposed a [[three-valued logic]] (True, False, Missing/[[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 =Christopher 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>