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{{about|a concept in relational database theory|function dependencies in the Haskell programming language|type class}}
{{refimprove|date=October 2012}}
In [[relational database]] theory, a '''functional dependency''' ('''FD''') is [[Relational database#Constraints|constraint]] between two attribute sets, whereby values in one set (the ''determinant'' set) determine the values of the other set (the ''dependent'' set). A functional dependency between a determinant set ''X'' and a dependent set ''
Given a [[Relation (database)|relation]] ''R'' and attribute sets ''X'',''Y'' <math>\subseteq</math> ''R'', ''X'' is said to functionally determine ''Y'' (written ''X'' → ''Y'') if each ''X'' value is associated with precisely one ''Y'' value. ''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]], that is, ''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>
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