Null is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database. Null was introduced by Codd as a method of representing missing data in the relational model. Codd also introduced a ternary ThreeValuedLogic, consisting of the truth values True, False, and Unknown, which is closely tied to the concept of Null. The Unknown truth value is generated whenever Null is compared with any data value, or with another Null.

More info in http://en.wikipedia.org/wiki/Null_%28SQL%29

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See NullVersusNone