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Definition
Let R(A 1,...,A n ) be a relation schema and Σ a set of functional dependencies over R(A 1,...,A n ). An attribute A i (i ∈{1,...,n}) is a prime attribute if A i is an element of some key of R(A 1,...,A n ). Then specification (R, Σ) is said to be in Second Normal Form (2NF) if for every nontrivial functional dependency X → A implied by Σ, it holds that A is a prime attribute or X is not a proper subset of any (candidate) key for R [1].
Key Points
In order to avoid update anomalies in database schemas containing functional dependencies, 2NF was introduced by Codd in [1]. This normal form is defined in terms of the notions of prime attribute and key 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 2NF since B is not a prime attribute and A is a proper subset of the key AC. On the other hand, (S(A, B, C), Γ) is in 2NF if Γ = {A → B, B → C}, since Ais a...
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Recommended Reading
Codd E.F. Further Normalization of the Data Base Relational Model. In Data base systems. Englewood Cliffs, N.J. Prentice-Hall, 1972, pp. 33–64.
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Arenas, M. (2009). Second Normal Form (2NF). In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_1263
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DOI: https://doi.org/10.1007/978-0-387-39940-9_1263
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