Relational model

The relational model for management of a database is a data model based on predicate logic and set theory.

Controversies

Codd himself proposed a three-valued logic version of the relational model, and a four-valued logic version has also been proposed, in order to deal with missing information. But these have never been implemented, presumably because of attending complexity. SQL NULLs were 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.

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