Synonyms
4NF
Definition
Let R(A1,…, An) be a relation schema and Σ a set of functional and multivalued dependencies over R(A1,…, An). Then (R, Σ) is said to be in fourth normal form (4NF) if for every nontrivial multivalued dependency X ↠ Y implied by Σ, it holds that X is a superkey for R.
Key Points
In order to avoid update anomalies in database schemas containing functional and multivalued dependencies, 4NF was introduced by Fagin in [2]. As for the case of BCNF, this normal form is defined in terms of the notion of superkey as shown above. For example, given a relation schema R(A, B, C) and a set of functional dependencies Σ = {A → B}, it does not hold that (R (A, B, C), Σ) is in 4NF since A ↠ B is a nontrivial multivalued dependency implied by Σ and A is not a superkey for R. Similarly, relation schema S(A, B, C) and set of multivalued dependencies Σ = {A ↠ C} is not in 4NF since A is not a superkey for S. On the other hand, relation schema T(A, B, C) and set of functional and...
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Beeri C. On the membership problem for functional and multivalued dependencies in relational databases. ACM Trans Database Syst. 1980;5(3):241–59.
Fagin R. Multivalued dependencies and a new normal form for relational databases. ACM Trans Database Syst. 1977;2(3):262–78.
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Arenas, M. (2018). Fourth Normal Form. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_9002
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DOI: https://doi.org/10.1007/978-1-4614-8265-9_9002
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