Re: Guessing?

From: Brian Selzer <brian_at_selzer-software.com>
Date: Wed, 9 Jul 2008 11:27:36 -0400
Message-ID: <JN4dk.13594$xZ.1754@nlpi070.nbdc.sbc.com>

"paul c" <toledobysea_at_ac.ooyah> wrote in message news:qXSck.59540$kx.32045_at_pd7urf3no...

> Brian Selzer wrote:
>> "paul c" <toledobysea_at_ac.ooyah> wrote in message 
>> news:NyRck.65245$Jx.43062_at_pd7urf1no...
>>> Brian Selzer wrote:

>>>> "paul c" <toledobysea_at_ac.ooyah> wrote in message
>>>> news:33Ock.59297$kx.264_at_pd7urf3no...
>>>>>> Brian Selzer wrote:
>>>>>> ...
>>>>>>> Consider the following statements:
>>>>>>>
>>>>>>> 1. Susan is an electrical engineer.
>>>>>>> 2. Susan is a mechanical engineer.
>>>>>>> 3. Susan is an electrical engineer or Susan is a mechanical 
>>>>>>> engineer.
>>>>>>> ...
>>>>> I have another question.  Suppose we have a database that involves 
>>>>> only statement 3.  Just what does its extension look like?
>>>>>

>>>> Correctly formed, each extension of the database would consist of two
>>>> relations: one for mechanical engineers and one for electrical
>>>> engineers. There are three possible extensions:
>>>>

>>>> one where Susan is both an electrical engineer and a mechanical
>>>> engineer,
>>>>

>>>> MechanicalEngineers {{Person:Susan}}
>>>> ElectricalEngineers {{Person:Susan}};
>>>> ...
>>> Okay, now stop right there.  (It is the case you originally gave, ie., 
>>> statements 1 and 2 are both true.)  What is the extension of 'statement' 
>>> 3?
>>
>> You've lost me.  When did I say that statements 1 and 2 are both true? 
>> IIRC, I wrote, "consider the following statements:"
>>
>>
>
>
> I see this snippet from June 22:
>
> "It is only the fact that the value that maps to Susan appears also in 
> both of the base relations that tells us that in fact Susan is both an 
> electrical engineer and a mechanical engineer."
>

The Susan I have in mind happens to be both an electrical and a mechanical engineer. But statement 3 would still be the case if she were either an electrical or a mechanical engineer but not both. If she were only an electrical engineer, then the extension of a relation representing statement 3 would still consist of that same tuple, {{Person:Susan}}. If she were only a mechanical engineer then the extension would still consist of that same tuple. In order be able to differentiate between the properties of being an electrical engineer and being a mechanical engineer you either need to add a flag attribute or use separate relations. Received on Wed Jul 09 2008 - 10:27:36 CDT

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