Fifth normal form

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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 every non-trivial join dependency in it is implied by the candidate keys.

A join dependency *{A, B, … Z} on R is implied by the candidate key(s) of R if and only if each of A, B, …, Z is a superkey for R.[1]

Example[edit]

Consider the following example:

Traveling Salesman Product Availability By Brand
Traveling Salesman Brand Product Type
Jack Schneider Acme Vacuum Cleaner
Jack Schneider Acme Breadbox
Willy Loman Robusto Pruning Shears
Willy Loman Robusto Vacuum Cleaner
Willy Loman Robusto Breadbox
Willy Loman Robusto Umbrella Stand
Louis Ferguson Robusto Vacuum Cleaner
Louis Ferguson Robusto Telescope
Louis Ferguson Acme Vacuum Cleaner
Louis Ferguson Acme Lava Lamp
Louis Ferguson Nimbus Tie Rack

The table's predicate is: Products of the type designated by Product Type, made by the brand designated by Brand, are available from the traveling salesman designated by Traveling Salesman.

In the absence of any rules restricting the valid possible combinations of Traveling Salesman, Brand, and Product Type, the three-attribute table above is necessary in order to model the situation correctly.

Suppose, however, that the following rule applies: A Traveling Salesman has certain Brands and certain Product Types in his repertoire. If Brand B1 and Brand B2 are in his repertoire, and Product Type P is in his repertoire, then (assuming Brand B1 and Brand B2 both make Product Type P), the Traveling Salesman must offer products of Product Type P those made by Brand B1 and those made by Brand B2.

In that case, it is possible to split the table into three:

Product Types By Traveling Salesman
Traveling Salesman Product Type
Jack Schneider Vacuum Cleaner
Jack Schneider Breadbox
Willy Loman Pruning Shears
Willy Loman Vacuum Cleaner
Willy Loman Breadbox
Willy Loman Umbrella Stand
Louis Ferguson Telescope
Louis Ferguson Vacuum Cleaner
Louis Ferguson Lava Lamp
Louis Ferguson Tie Rack
Brands By Traveling Salesman
Traveling Salesman Brand
Jack Schneider Acme
Willy Loman Robusto
Louis Ferguson Robusto
Louis Ferguson Acme
Louis Ferguson Nimbus
Product Types By Brand
Brand Product Type
Acme Vacuum Cleaner
Acme Breadbox
Acme Lava Lamp
Robusto Pruning Shears
Robusto Vacuum Cleaner
Robusto Breadbox
Robusto Umbrella Stand
Robusto Telescope
Nimbus Tie Rack


In this case, it's impossible for Louis Ferguson to refuse to offer Vacuum Cleaners made by ACME (assuming ACME makes Vacuum Cleaners) if he sells anything else made by Acme (Lava Lamp) and he also sells Vacuum Cleaners made by any other brand (Robusto).

Note how this setup helps to remove redundancy. Suppose that Jack Schneider starts selling Robusto's products Breadboxes and Vacuum Cleaners. In the previous setup we would have to add two new entries one for each product type (<Jack Schneider, Robusto, Breadboxes>, <Jack Schneider, Robusto, Vacuum Cleaners>). With the new setup we need to add only a single entry (<Jack Schneider, Robusto>)in Brands By Traveling Salesman.

Usage[edit]

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.

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Further reading[edit]