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4. Database Design Principles > 4.7. Boyce-Codd Normal Form

Boyce-Codd Normal Form

It is possible to find table schemes that are in third normal form, but still have redundancy. Here is an example.

Consider the table scheme {City,StreetName,ZipCode}, with dependencies:

{City,StreetName} {ZipCode}

and:

{ZipCode} {City}

(Although in real life, a zip code may be shared by two different cities, we will assume otherwise for the purposes of illustration.) This table scheme is in third normal form. To see this, observe that the keys are {City,StreetName} and {ZipCode,StreetName}. Hence, no attribute is strictly informational, and there is nothing to violate third normal form.

On the other hand, consider Table 4-4. We can fill in the blank city name because {ZipCode}{City}.


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