Information about Superkey

A superkey is defined in the relational model of database organisation 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.

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}
• {Monarch Name, Monarch Number, Royal House}

See also

• compound key
• candidate key
• primary key
• alternate key
• foreign key

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