Fifth normal form

Fifth normal form (5NF), also known as Project-join normal form (PJ/NF) is a level of database normalization, designed to reduce redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships. A table is said to be in the 5NF if and only if it is in 4NF and every join dependency in it is implied by the candidate keys.

Example

Consider the following example:

Note how this setup helps to remove redundancy. Suppose that Jack Schneider starts selling Robusto's products. In the previous setup we would have to add two new entries since Jack Schneider is able to sell two Product Types covered by Robusto: Breadboxes and Vacuum Cleaners. With the new setup we need only add a single entry (in Brands By Travelling Salesman).

Usage

Only in rare situations does a 4NF table not conform to 5NF. These are situations in which a complex real-world constraint governing the valid combinations of attribute values in the 4NF table is not implicit in the structure of that table. If such a table is not normalized to 5NF, the burden of maintaining the logical consistency of the data within the table must be carried partly by the application responsible for insertions, deletions, and updates to it; and there is a heightened risk that the data within the table will become inconsistent. In contrast, the 5NF design excludes the possibility of such inconsistencies. Spurious rows in result set may occur unless you re-join ALL of the tables in 5NF.

ee also

*Attribute-value system
*Sixth normal form

References

* [http://www.datamodel.org/NormalizationRules.html Rules Of Data Normalization]
* Kent, W. (1983) " [http://www.bkent.net/Doc/simple5.htm A Simple Guide to Five Normal Forms in Relational Database Theory] ", Communications of the ACM, vol. 26, pp. 120-125
* Date, C.J., & Darwen, H., & Pascal, F. " [http://www.dbdebunk.com Database Debunkings] "
* [http://www.utexas.edu/its/windows/database/datamodeling/rm/rm8.html Advanced Normalization]