Re: Guessing?

paul c wrote:

> Brian Selzer wrote:
>

>> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message 
>> news:48738203$0$4049$9a566e8b_at_news.aliant.net...
>>
>>> paul c wrote:
>>>
>>>> Brian Selzer wrote:
>>>>
>>>>> "paul c" <toledobysea_at_ac.ooyah> wrote in message
>>>>
>>>> ...
>>>>
>>>>>> However, I'm content to say that all three relations have the same 
>>>>>> predicate, assuming no attribute renaming is involved in your 
>>>>>> interpretation. I know many people say that the 'or' is introduced 
>>>>>> to the predicate of R.  I don't believe there is any law or 
>>>>>> principle, including relational closure, that requires anybody to 
>>>>>> think this way.
>>>>>>
>>>>> How can they have the same predicate if they can have different 
>>>>> extensions? That doesn't make any sense.
>>>>> ...
>>>>
>>>> They are misconceived.  The example strikes me as akin to Joe C's 
>>>> word games.
>>>
>>> To answer Selzer's query, they have different external predicates but 
>>> the same predicate as far as the DBMS can calculate.
>>
>>
>> That doesn't make sense either.  If there are different external 
>> predicates, then shouldn't that be reflected by there being different 
>> relation names, and thus differing internal predicates?

Usually when discussing the composition operator. f(g(x)) sort of thing.

> (Surely relation names aren't anything but an implementation device.)

I don't think I entirely agree. Received on Tue Jul 08 2008 - 20:08:48 CEST