Second normal form (2NF) is a normal form used in database normalization. 2NF was originally defined by E.F. Codd in 1971.
A table that is in first normal form (1NF) must meet additional criteria if it is to qualify for second normal form. Specifically: a table is in 2NF if and only if it is in 1NF and no non prime attribute is dependent on any proper subset of any candidate key of the table. A non prime attribute of a table is an attribute that is not a part of any candidate key of the table.
Put simply, a table is in 2NF if and only if it is in 1NF and every non-prime attribute of the table is either dependent on the whole of a candidate key, or on another non prime attribute.
Note that when a 1NF table has no composite candidate keys (candidate keys consisting of more than one attribute), the table is automatically in 2NF.
Read more about Second Normal Form: Example
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