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.
Related Resources:
Database
Design
Visit our database design subject page for more on normal forms and
normalization
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