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   The choice of the fields to insert in each single file is very important
   for the correct development of an application. Inserting a field in the
   wrong file can cause redundant information and create an inconsistency
   among the data within the database.

   Let's consider the following example:

   A database contains the articles present in a sales warehouse. These
   articles can be divided into categories, among which, for example,
   hardware and software.

   Each article is identified by its name, the available quantity, the
   category, and the applicable discount rate. Suppose that the company's
   decision is to set a specific discount for an entire category (ex.
   software).

   Let's now suppose that we defined a single file, Art, in which all the
   informations regarding the articles is saved, including the discount to
   apply.

   Let's say that the discount for software articles, initially set to 20%,
   is reduced by the commercial department of the company to 10%. In this
   case, all the records corresponding to the articles for that category
   would have to be modified.

   If, instead, we did define two files among which to divide the information
   specific to each article (Art) and to each category (Cat), the problem
   would have been solved by modifying one single record of the Cat file.

   The normalisation theory of the relational databases offers a group of
   rules that help to choose the correct file format. These rules define a
   group of properties, called normal forms, that each file must maintain
   in order to guarantee a correct behaviour of the application.

   The application behaves correctly if the data consistency is preserved
   after each insertion, modification or deletion of data. The database is
   consistent if all rules, expressed or implied, are respected.

   For instance, as it has been decided that all articles of the same
   category must have the same discount rate, a change of discount rate in
   one article would create an inconsistency in the whole database.

   Different types of normal forms exist, each imposing restrictions on the
   file format. We will examine the three most common ones.

   The construction of a database whose files satisfy these restrictions is a
   necessary condition in order to assure the consistency of a company's
   data. Please pay close attention to this part of the development phase of
   an application.