Second Normal Form (2NF)

Synonyms

2NF

Definition

Let R(A₁,…, A_n) be a relation schema and Σ a set of functional dependencies over R(A₁,…, A_n). An attribute A_i (i ∈{1,…,n}) is a prime attribute if A_i is an element of some key of R(A₁,…, 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 key (and thus it...