^ Introduction to Database Management Systems.In other words, what the paper calls a primary key is what later (and better) became known as a superkey, and what the paper calls a nonredundant (better: irreducible) primary key is what later became known as a candidate key or (better) just a 'key'. Note that the extract allows a 'relation' to have any number of primary keys, and moreover that such keys are allowed to be 'redundant' (better: reducible). 'Codd's First Relational Papers: A Critical Analysis' (PDF). A superkey defines a functional dependency constraint of a relation schema which must hold for all possible instance relations of that relation schema. In reality, superkeys cannot be determined simply by examining one set of tuples in a relation. įor example, in an employee schema with attributes employeeID, name, job, and departmentID, if employeeID values are unique then employeeID combined with any or all of the other attributes can uniquely identify tuples in the table. A candidate key (or minimal superkey) is a superkey that can't be reduced to a simpler superkey by removing an attribute.

Tuples in a relation are by definition unique, with duplicates removed after each operation, so the set of all attributes is always uniquely valued for every tuple. The set of all attributes is always a superkey (the trivial superkey). That is, non-key attributes are functionally dependent on the superkey. Because superkey values are unique, tuples with the same superkey value must also have the same non-key attribute values. In the relational data model a superkey is a set of attributes that uniquely identifies each tuple of a relation. For the keyboard button, see Super key (keyboard button). This article is about the database design term.

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