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What is second normal form?

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Second normal form (2NF) is a normal form used in database normalization. 2NF was originally defined by E.F. Codd in 1971.[1]

To qualify for second normal form a relation must:

(1) be in first normal form (1NF)

(2) not have any non-prime attribute that is dependent on any proper subset of any candidate key of the relation. A non-prime attribute of a relation is an attribute that is not a part of any candidate key of the relation.

Put simply, a relation is in 2NF if it is in 1NF and every non-prime attribute of the relation is dependent on the whole of every candidate key. Note that it does not put any restriction on the non-prime to non-prime attribute dependency. That is addressed in Third normal form.
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