Editing Superkey (section)

==Example==
{| class="wikitable"
|+ English Monarchs
|-
! Monarch Name
! Monarch Number
! Royal House
|-
| Edward
| II
| Plantagenet
|-
| Edward
| III
| Plantagenet
|-
| Richard
| III
| Plantagenet
|-
| Henry
| IV
| Lancaster
|}

First, list out all the sets of attributes:
:• {}
:• {Monarch Name} &nbsp;
:• {Monarch Number} &nbsp;
:• {Royal House}
:• {Monarch Name, Monarch Number}
:• {Monarch Name, Royal House}
:• {Monarch Number, Royal House}
:• {Monarch Name, Monarch Number, Royal House}

Second, eliminate all the sets which do not meet superkey's requirement.  For example, {Monarch Name, Royal House} cannot be a superkey because for the same attribute values (Edward, Plantagenet), there are two distinct tuples:
* (Edward, '''II''', Plantagenet)
* (Edward, '''III''', Plantagenet)

Finally, after elimination, the remaining sets of attributes are the only possible superkeys in this example:
* {Monarch Name, Monarch Number} — this is also the [[candidate key]]
* {Monarch Name, Monarch Number, Royal House} — this is also the trivial superkey.

In reality, superkeys cannot be determined simply by examining one set of tuples in a relation. A superkey defines a [[functional dependency]] constraint of a relation schema which must hold for all possible instance relations of that relation schema.