Re: 4NF is Where It Is At! [WAS Re: 1:1 relationships]

David Cressey wrote:
> Hmm...
>
>
> how about the following situation, as defined by the (hypothetical) phone
> company:
>
> "You can take your phone number with you when you move, if and only if your
> new address is served by the same CO (Central Office?) as the old address
> was served by.
>
> If, however, your new address is served by a different CO, then you may not
> take your phone number with you."
>
> Now what are the implications for dependency, and the corresponding
> implications for normalization?

Interesting example. What you describe is a dynamic database constraint. But normalization usually only deals with static database constraints. Because the constraint does not imply any new functional dependencies (unless you introduce a column CO), it has no impact on the normalization process.

-- 
  Jan Hidders