Re: two nasty schemata, union types and surrogate keys

On Sep 25, 6:04 am, Roy Hann <specia..._at_processed.almost.meat> wrote:
> Brian wrote:
> > What does the closed world assumption have to do with violating 5NF?
>
> You can't have a violation of 5NF without it.
>
> --
> Roy

Yes, you can. Suppose that you have the scheme

{Supplier, SupplierPhone, PartNumber}

which satisfies the functional dependencies

PartNumber --> Supplier --> SupplierPhone

This is a violation of 5NF because it is a violation of 3NF. Under the open world interpretation, what is in the database is what is known to be true, but that doesn't change the fact that if you delete the row with the only part known to be supplied by a particular supplier, you also erase the knowledge that that particular supplier has a particular phone number. Received on Fri Sep 25 2009 - 13:35:11 CEST