Third Normal Form (3NF)

Definition: In order to be in Third Normal Form, a relation must first fulfill the requirements to be in Second Normal Form.  Additionally, all attributes that are not dependent upon the primary key must be eliminated.

Examples: The following table is NOT in Third Normal Form:

Company	City	State	ZIP
Acme Widgets	New York	NY	10169
ABC Corporation	Miami	FL	33196
XYZ, Inc.	Columbia	MD	21046

In this example, the city and state are dependent upon the ZIP code.  To place this table in 3NF, two separate tables would be created -- one containing the company name and ZIP code and the other containing city, state, ZIP code pairings.

This may seem overly complex for daily applications and indeed it may be.  Database designers should always keep in mind the tradeoffs between higher level normal forms and the resource issues that complexity creates.