Codd's 12 Rules
Codd's twelve rules are a set of thirteen rules (numbered zero to twelve) proposed by Edgar F. Codd, a pioneer of the relational model for databases, designed to define what is required from a database management system in order for it to be considered relational, i.e., a relational database management system (RDBMS). They are sometimes jokingly referred to as "Codd's Twelve Commandments".
Codd produced these rules as part of a personal campaign to prevent his vision of the relational database being diluted, as database vendors scrambled in the early 1980s to repackage existing products with a relational veneer. Rule 12 was particularly designed to counter such a positioning.
Even if such repackaged non-relational products eventually gave way to SQL DBMSs, no popular "relational" DBMSs are actually relational, be it by Codd’s twelve rules or by the more formal definitions in his papers, in his books or in succeeding works in the academia or by its coworkers and successors, Christopher J. Date, Hugh Darwen, David McGoveran and Fabian Pascal. Only less known DBMSs, most of them academic, strive to comply. The only commercial example, as of December 2010, is Dataphor.
Some rules are controversial, especially rule three, due to the debate on three-valued logic.
Read more about Codd's 12 Rules: The Rules
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