Superkey

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Superkey

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This article is about the programming term. For the keyboard button, see Super key (keyboard button).
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It has been suggested that this article or section be merged into unique key. (Discuss)

A superkey is defined in the relational model of database organization as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. Equivalently a superkey can also be defined as a set of attributes of a relvar upon which all attributes of the relvar are functionally dependent.

Note that if attribute set K is a superkey of relvar R, then at all times it is the case that the projection of R over K has the same cardinality as R itself.

Informally, a superkey is a set of columns within a table whose values can be used to uniquely identify a row. A candidate key is a minimal set of columns necessary to identify a row, this is also called a minimal superkey. For example, given an employee table, consisting of the columns employeeID, name, job, and departmentID, we could use the employeeID in combination with any or all other columns of this table to uniquely identify a row in the table. Examples of superkeys in this table would be {employeeID, Name}, {employeeID, Name, job}, and {employeeID, Name, job, departmentID}.

In a real database we don't need values for all of those columns to identify a row. We only need, per our example, the set {employeeID}. This is a minimal superkey – that is, a minimal set of columns that can be used to identify a single row. So, employeeID is a candidate key.
[edit] Example
English Monarchs Monarch Name Monarch Number Royal House
Edward II Plantagenet
Edward III Plantagenet
Richard II Plantagenet
Henry IV Lancaster

In this example, the possible superkeys are:

* {Monarch Name, Monarch Number} (Candidate Key)
* {Monarch Name, Monarch Number, Royal House}

[edit] External links

* Relation Database terms of reference, Keys: An overview of the different types of keys in an RDBMS

[edit] See also

* Alternate key
* Compound key
* Foreign key
* Primary key

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Database · ACID · CRUD · Null · Candidate key · Foreign key · Primary key · Superkey · Surrogate key · Armstrong's axioms
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* This page was last modified on 25 April 2010 at 16:47.
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