Database Normalization: 5th Normal Form and Beyond

This article follows on from the 4th normal form article.

There are normal forms beyond 4th that are mainly of academic interest, as the problems they exist to solve rarely appear in practice. This series won't discuss then in detail, but for those interested, the following example provides a taste.

The sales rep example

Sales rep	Company	Product
Felicia Powers	Exclusive	Books
Afzal Ignesund	Wordsworth	Magazines
Felicia Powers	Exclusive	Magazines

Usually you would store this data in one table, as you need all three records to see which combinations are valid. Afzal Ignesund sells magazines for Wordsworth, but not necessarily books. Felicia Powers happens to sell both books and magazines for Exclusive. However, let's add another condition. If a sales rep sells a certain product, and they sell it for a particular company, then they must sell that product for that company.

Let's look at a larger data set adhering to this condition:

Looking at a larger set of data

Sales rep	Company	Product
Felicia Powers	Exclusive	Books
Felicia Powers	Exclusive	Magazines
Afzal Ignesund	Wordsworth	Books
Felicia Powers	Wordsworth	Books
Felicia Powers	Wordsworth	Magazines

Now, with this extra dependency, you could normalize the table above into three separate tables without losing any facts, as shown below:

Creating a table with Sales rep and Product

Sales rep	Product
Felicia Powers	Books
Felicia Powers	Magazines
Afzal Ignesund	Books

Creating a table with Sales rep and Company

Sales rep	Company
Felicia Powers	Exclusive
Felicia Powers	Wordsworth
Afzal Ignesund	Wordsworth

Creating a table with Company and Product

Company	Product
Exclusive	Books
Exclusive	Magazines
Wordsworth	Books
Wordsworth	Magazines

Basically, a table is in 5th normal form if it cannot be made into any smaller tables with different keys (most tables can obviously be made into smaller tables with the same key!).

Beyond 5th normal form you enter the heady realms of domain key normal form, a kind of theoretical ideal. Its practical use to a database designer os similar to that of infinity to a bookkeeper - i.e. it exists in theory but is not going to be used in practice. Even the most demanding owner is not going to expect that of the bookkeeper!

For those interested in pursuing this academic and highly theoretical topic further, I suggest obtaining a copy of An Introduction to Database Systems by C.J. Date, at the time of writing in its 8th edition, or Relational Theory for Computer Professionals by the same author.

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