Many databases require that you plan your tables so they can handle multiple occurrences of the same type of item.
A book, for example, might have several authors. The following table shows one way to handle such a circumstance:

The Author1 and Author2 fields are able to handle up to two authors for each book, but there are two obvious problems with the table:

1. If a book has only one author, the Author2 column is wasted space.
   
2. There is no place to store the name of a third, fourth, or succeeding author.

There is also a problem from a theoretical standpoint. One of the rules of tables is that every column in a table must represent a unique attribute of an entity. In the case of the Books table, the Author1 and Author2 columns represent the same attribute: an author. The technical term for columns that represent the same attribute is a repeating group. (Don't let the semantics fool you--the lead and second authors of a book are separate individuals, but they are both members of the set of Authors.)
For a table to be in first normal form (1NF), the table must not contain any repeating groups.
first normal form (1NF) : A table is in first normal form if it contains no repeating groups.
The next lesson describes how to decompose a table to first normal form.