Domain/key normal form
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Domain/key normal form (or DKNF) a normal form used in database normalization which requires that the database contains no constraints other than domain constraints and key constraints.
A domain constraint specifies the permissible values for a given attribute, while a key constraint specifies the attributes that uniquely identify a row in a given table.
A violation of DKNF occurs in the following table:
| Wealthy Person | Wealthy Person Type | Net Worth in Dollars |
|---|---|---|
| Steve | Eccentric Millionaire | 124,543,621 |
| Roderick | Evil Billionaire | 6,553,228,893 |
| Katrina | Eccentric Billionaire | 8,829,462,998 |
| Gary | Evil Millionaire | 495,565,211 |
(Assume that the domain for Wealthy Person consists of the names of all wealthy people in a pre-defined sample of wealthy people; the domain for Wealthy Person Type consists of the values 'Eccentric Millionaire', 'Eccentric Billionaire', 'Evil Millionaire', and 'Evil Billionaire'; and the domain for Net Worth in Dollars consists of all integers greater than or equal to 1,000,000.)
There is a constraint linking Wealthy Person Type to Net Worth in Dollars, even though we cannot deduce one from the other. The constraint dictates that an Eccentric Millionaire or Evil Millionaire will have a net worth of 1,000,000 to 999,999,999 inclusive, while an Eccentric Billionaire or Evil Billionaire will have a net worth of 1,000,000,000 or higher. This constraint is neither a domain constraint nor a key constraint; therefore we cannot rely on domain contraints and key constraints to guarantee that an inconsistent Wealthy Person Type / Net Worth in Dollars combination does not make its way into the database.
The DKNF violation could be eliminated by altering the Wealthy Person Type domain to make it consist of just two values, 'Evil' and 'Eccentric' (the wealthy person's status as a millionaire or billionaire is implicit in their Net Worth in Dollars, so no useful information is lost).
DKNF is frequently difficult to achieve in practice.
[edit] References
[edit] External links
- Database Normalization Basics by Mike Chapple (About.com)
- An Introduction to Database Normalization by Mike Hillyer.
- Normalization by ITS, University of Texas.
- Rules of Data Normalization by Data Model.org
- A tutorial on the first 3 normal forms by Fred Coulson
- Free PDF poster available by Marc Rettig
- Description of the database normalization basics by Microsoft
| Topics in Database normalization |
|
First normal form | Second normal form | Third normal form |
