<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> In relational database theory, a functional dependency is a constraint between two sets of attributes in a relation from a database . In other words, functional dependency is a constraint that describes the relationship between attributes in a relation . </P> <P> 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 π X, Y R (\ displaystyle \ pi _ (X, Y) R) is a function, i.e. Y is a function of X . 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 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 . </P> <P> 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 . </P>

When is a functional dependency said to be trivial
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