Editing Third normal form (section)

{{Short description|Normalizing a database design to reduce the duplication of data and ensure referential integrity}}
'''Third normal form''' ('''3NF''') is a [[database schema]] design approach for [[relational database]]s which uses [[Database normalization|normalizing]] principles to reduce the duplication of data, avoid [[data anomaly|data anomalies]], ensure [[referential integrity]], and simplify [[data management]]. It was defined in 1971 by [[Edgar F. Codd]], an English computer scientist who invented the [[relational model]] for [[database]] management.

A [[Relation (database)|database relation]] (e.g. a [[Table (database)|database table]]) is said to meet third normal form standards if all the attributes (e.g. [[Column (database)|database columns]]) are [[Functional dependency|functionally dependent]] on solely a [[Candidate key|key]], except the case of functional dependency whose right hand side is a prime attribute (an attribute which is strictly included into some key). Codd defined this as a relation in [[second normal form]] where all non-prime attributes depend only on the [[candidate key]]s and do not have a [[transitive dependency]] on another key.<ref>Codd, E. F. "Further Normalization of the Data Base Relational Model", p. 34.</ref> 

A hypothetical example of a failure to meet third normal form would be a hospital database having a table of patients which included a column for the telephone number of their doctor. (The phone number is dependent on the doctor, rather than the patient, thus would be better stored in a table of doctors.) The negative outcome of such a design is that a doctor's number will be duplicated in the database if they have multiple patients, thus increasing both the chance of input error and the cost and risk of updating that number should it change (compared to a third normal form-compliant data model that only stores a doctor's number once on a doctor table).

Codd later realized that 3NF did not eliminate all undesirable data anomalies and developed a stronger version to address this in 1974, known as [[Boyce–Codd normal form]].