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   The second normal form involves the primary key of the file. This rule
   describes how the primary key must be chosen.

   A file is in second normal form if its fields depend functionally and
   completely from the primary key.

   Functional dependence means that each value of the key must correspond
   to only one value of every other field. Let's consider the following
   example:

   The key must be selected in such a way, that two records may not have the
   same value for the key. This means that the key serves to identify each
   record uniquely.

   A valid key value, for instance, could be CatCode+ArtCode. In fact,
   this couple of values identifies uniquely a given article.

   Complete dependence means that it cannot be possible to build a second
   key, made up of a subset of fields contained in the first key in such a
   way, that those fields functionally depend on the second key.

   The key chosen in the previous example, CatCode+ArtCode, does not
   follow this rule. In fact, as can be deduced from the example, the
   article's code (ArtCode) alone is sufficient to uniquely identify a
   record.

   The correct key, in order to satisfy the second normal form, is made up of
   the ArtCode field alone.