Re: two nasty schemata, union types and surrogate keys

From: Roy Hann <specially_at_processed.almost.meat>
Date: Fri, 25 Sep 2009 08:40:02 -0500
Message-ID: <D-WdnZKSWaqvVSHXnZ2dnUVZ8g2dnZ2d_at_pipex.net>



Brian wrote:

> 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.

I stupidly assumed you'd consider the comparison with a table that *is* in 4NF.

-- 
Roy
Received on Fri Sep 25 2009 - 08:40:02 CDT

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