Second normal form (2NF) is a normal form used in database normalization. 2NF was originally defined by E.F. Codd in 1971.
A relation that is in first normal form (1NF) must meet additional criteria if it is to qualify for second normal form. Specifically: a relation is in 2NF if it is in 1NF and no non-prime attribute 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.
Video Second normal form
2NF and candidate keys
A functional dependency on part of any candidate key is a violation of 2NF. In addition to the primary key, the relation may contain other candidate keys; it is necessary to establish that no non-prime attributes have part-key dependencies on any of these candidate keys.
Multiple candidate keys occur in the following relation:
Even if the designer has specified the primary key as {Model Full Name}, the relation is not in 2NF because of the other candidate keys. {Manufacturer, Model} is also a candidate key, and Manufacturer Country is dependent on a proper subset of it: Manufacturer. To make the design conform to 2NF, it is necessary to have two relations:
Maps Second normal form
See also
- Attribute-value system
References
Further reading
External links
- Database Normalization Basics by Mike Chapple (About.com)
- An Introduction to Database Normalization by Mike Hillyer.
- A tutorial on the first 3 normal forms by Fred Coulson
- Description of the database normalization basics by Microsoft
Source of the article : Wikipedia
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