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Definition
Given a relation schema R[U], a functional dependency (FD) is an expression of the form X → Y , where X,Y ⊆ U. An instance I of R[U] satisfies X → Y , denoted by I⊧X → Y , if for every two tuples t 1,t 2 in I, t 1[X] = t 2[X] implies t 1[Y ] = t 2[Y ]. That is, whenever two tuples contain the same values for attributes in X, they must have the same values for attributes in Y. A functional dependency X → Y is called trivial if Y ⊆ X.
A key dependency is a functional dependency of the form X → U. Then X is called a superkey for relation R. If there is no proper subset Y of X such that Y is a superkey, then X is called a key.
Key Points
Functional dependencies form an important class of integrity constraints that play a critical role in maintaining the integrity of data, query optimization and indexing, and especially schema design. The focus of the normalizationtechnique in schema design is on avoiding redundancies caused by functional and other types of...
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Recommended Reading
Abiteboul S., Hull R., Vianu V. Foundations of Databases. Addison-Wesley, Reading, MA, USA, 1995.
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© 2009 Springer Science+Business Media, LLC
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Kolahi, S. (2009). Functional Dependency. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_1247
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