Third normal form

Third normal form (3NF) is a normal form that is used in normalizing a database design to reduce the duplication of data and ensure referential integrity by ensuring that: Third normal form (3NF) is a normal form that is used in normalizing a database design to reduce the duplication of data and ensure referential integrity by ensuring that: (1) the entity is in second normal form (2) no non-prime (non-key) attribute is transitively dependent on any key i.e. no non-prime attribute depends on other non-prime attributes. All the non-prime attributes must depend only on the keys. 3NF was designed to: eliminate undesirable data anomalies; reduce the need for restructuring over time; make the data model more informative; make the data model neutral to different kinds of query statistics. Codd later realized that 3NF did not achieve the first of these goals and developed Boyce-Codd Normal Form to address the limitations of 3NF. The third normal form (3NF) is a normal form used in database normalization. 3NF was originally defined by E.F. Codd in 1971. Codd's definition states that a table is in 3NF if and only if both of the following conditions hold: A non-prime attribute of R is an attribute that does not belong to any candidate key of R. A transitive dependency is a functional dependency in which X → Z (X determines Z) indirectly, by virtue of X → Y and Y → Z (where it is not the case that Y → X). A 3NF definition that is equivalent to Codd's, but expressed differently, was given by Carlo Zaniolo in 1982. This definition states that a table is in 3NF if and only if, for each of its functional dependencies X → A, at least one of the following conditions holds: Zaniolo's definition gives a clear sense of the difference between 3NF and the more stringent Boyce–Codd normal form (BCNF). BCNF simply eliminates the third alternative ('Every element of A-X, the set difference between A and X, is a prime attribute'). An approximation of Codd's definition of 3NF, paralleling the traditional pledge to give true evidence in a court of law, was given by Bill Kent: ' non-key must provide a fact about the key, the whole key, and nothing but the key.' A common variation supplements this definition with the oath: 'so help me Codd'.