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The ISO SQL implementation of Null is the subject of criticism, debate and calls for change. In ''The Relational Model for Database Management: Version 2'', Codd suggested that the SQL implementation of Null was flawed and should be replaced by two distinct Null-type markers. The markers he proposed were to stand for ''"Missing but Applicable"'' and ''"Missing but Inapplicable"'', known as ''A-values'' and ''I-values'', respectively. Codd's recommendation, if accepted, would have required the implementation of a four-valued logic in SQL.<ref name="isbn0201141922">{{cite book|last=Codd|first=E.F.|year=1990|title=The Relational Model for Database Management|edition=Version 2|publisher=[[Pearson PLC|Addison Wesley Publishing Company]]|isbn=978-0-201-14192-4}}</ref> Others have suggested adding additional Null-type markers to Codd's recommendation to indicate even more reasons that a data value might be "Missing", increasing the complexity of SQL's logic system. At various times, proposals have also been put forth to implement multiple user-defined Null markers in SQL. Because of the complexity of the Null-handling and logic systems required to support multiple Null markers, none of these proposals have gained widespread acceptance.
[[Chris Date]] and [[Hugh Darwen]], authors of ''
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* [[RDBMS|Relational database management system]]
* [[Join (SQL)]]
==References==
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