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the functipnal dependency is actually a bijective function. The set of tuples the projection returned gives us a bijecive function structure. |
just correcting an edit i made before. My fault. restored it to what it was before. |
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{{refimprove|date=October 2012}}
In [[relational database]] theory, a '''functional dependency''' is a '''[[Relational database#Constraints|constraint]]''' between two sets of attributes in a [[Relation (database)|relation]] from a database. In other words, a functional dependency is a constraint between two keys.
Given a relation ''R'' and sets of attributes <math>X,Y \subseteq R</math>, ''X'' is said to '''functionally determine''' ''Y'' (written ''X'' → ''Y'') if and only if each ''X'' value in ''R'' is associated with precisely one ''Y'' value in ''R''; ''R'' is then said to ''satisfy'' the functional dependency ''X'' → ''Y''. Equivalently, the [[projection (relational algebra)|projection]] <math>\Pi_{X,Y}R</math> is a [[
In other words, a dependency FD: ''X'' → ''Y'' means that the values of ''Y'' are determined by the values of ''X''. Two tuples sharing the same values of ''X'' will necessarily have the same values of ''Y''.
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