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{{about|a concept in relational database theory|functional dependencies in the Haskell programming language|type class}}
{{refimprove|date=October 2012}}
In [[relational database]] theory, a '''functional dependency''' is a '''[[Relational database#Constraints|constraint]]''' between two
Given a relation ''R'', a set of attributes ''X'' in ''R'' is said to '''functionally determine''' another set of attributes ''Y'', also in ''R'', (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 [[function (mathematics)|function]], i.e. ''Y'' is a function of ''X''.<ref name="HalpinMorgan2008">{{cite book |author1=Terry Halpin |title=Information Modeling and Relational Databases |url=https://books.google.com/books?id=puO_VlbR_x4C&pg=PA140 |year=2008 |publisher=Morgan Kaufmann |isbn=978-0-12-373568-3 |page=140 |edition=2nd}}</ref><ref name="Date2012">{{cite book |author=Chris Date |title=Database Design and Relational Theory: Normal Forms and All That Jazz |url=https://books.google.com/books?id=8jAGhpMSjAcC&pg=PA21 |year=2012 |publisher=O'Reilly Media, Inc. |isbn=978-1-4493-2801-6 |page=21}}</ref> In simple words, if the values for the ''X'' attributes are known (say they are ''x''), then the values for the ''Y'' attributes corresponding to ''x'' can be determined by looking them up in ''any'' [[Tuple#Relational model|tuple]] of ''R'' containing ''x''. Customarily ''X'' is called the ''determinant'' set and ''Y'' the ''dependent'' set. A functional dependency FD: ''X'' → ''Y'' is called ''trivial'' if ''Y'' is a [[subset]] of ''X''.
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