The definition of first, second, third, fourth and fifth normal forms.

By: Borland Staff

Abstract: Discussion of Database Design

Problem:
The definition of first, second, third, fourth and fifth normal forms.

Solution:
In the process of efficiently storing data, and eliminating redundancy,  tables in a database are 
designed and created to be in one of five possible normal forms.  Each normal form contains and 
enforces the rules of the previous form,  and,  in turn,  applies some stricter rules on the design of 
tables.

A set of tables in a database are initially said to be in 0 normal form.


First Normal Form:
===============
  Tables are said to be in first normal form when:
  - The table has a primary key.
  - No single attribute (column) has multiple values.
  - The non-key attributes (columns) depend on the primary key.

  Some examples of placing a table in first normal form are:

  author_id:		stories:
  000024			novelist, playwright		// multiple values
  000034			magazine columnist
  002345			novella, newpaper columnist	// multiple values

  In first normal form the table would look like:

  author_id:		stories:
  000024			novelist
  000024			playwright
  000034			magazine columnist
  002345			novella
  002345			newpaper columnist


Second Normal Form:
=================
  Tables are said to be in second normal form when:
  - The tables meet the criteria for first normal form.
  - If the primary key is a composite of attributes (contains multiple 
    columns), the non key attributes (columns) must depend on the whole
    key.

  Note: Any table with a primay key that is composed of a single 
  attribute (column) is automatically in second normal form.

Third Normal Form:
================
  Tables are said to be in third normal form when:
  - The tables meet the criteria for second normal form.
  - Each non-key attribute in a row does not depend on the entry in 
    another key column.

Fourth Normal Form:
================
  Tables are said to be in fourth normal form when:
  - The table meets the criteria for third normal form.
  - Situations where non-key attributes depend on the key column 
    exclusive of other non-key columns are eliminated.


Fifth Normal Form:
===============
  Tables are said to be in fifth normal form when:
  - The table meets the criteria for fourth normal form.
  - The table consists of a key attribute and a non-key attribute only.

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