Re: Guessing?
Date: Wed, 9 Jul 2008 11:27:36 -0400
Message-ID: <JN4dk.13594$xZ.1754_at_nlpi070.nbdc.sbc.com>
"paul c" <toledobysea_at_ac.ooyah> wrote in message news:qXSck.59540$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 - 17:27:36 CEST