Re: A Logical Model for Lists as Relations

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Thu, 11 May 2006 13:51:20 GMT
Message-ID: <sjH8g.6281$A26.160107@ursa-nb00s0.nbnet.nb.ca>

JOG wrote:

> Bob Badour wrote:
>

>>Jay Dee wrote:
>>
>>>Bob Badour wrote:
>>> > Since it is relatively easy to write a query that extends a relation
>>> > with a rank per any explicit order, I am not even sure the ordinal
>>> > attribute is required.
>>>
>>>True.  I wasn't exactly sure what MS wanted to happen, for example,
>>>to the fourth element in a list when the second element is elided.
>>>Does it become the third?  If so, your proposal is a good
>>>solution.  But...
>>>
>>>What if he envisions a list in which duplication of elements is
>>>significant?
>>
>>If one has a numeric index that differs for each tuple, one never has
>>duplication. If one has duplication, one wonders how to refer to the
>>duplicates. As Codd observed long ago, once one has said a thing is
>>true, what does saying it again achieve?

>
>
> Wise words from the Codd. However, I think people confuse (and by
> people I essentially mean myself) trying to encode statements such as:
>
> There is a person called Ernie who likes beer.
> There is another person called Ernie who also likes beer.
>
> Ex likes(x, Ernie, beer) &&
> Ey likes(y, Ernie, beer) &&
> x != y
>
> The two Ernies are obviously different people but I don't have enough
> information to harness liebniz equality and so distinguish them, I just
> have the fluff of 'another' saying that those distinctions do exist. So
> I obviously use a surrogate to represent the missing information.
> Something seems sliiightly artificial about this though - I just can't
> put my finger on what at the moment. (perhaps in terms of logic I
> should just be encoding "There are 2 people called Ernie who like
> Beer"?).

If there are two Ernies in real life, there is some way to distinguish them. The problem you describe is not a flaw of the logical data model but an incomplete analysis and design.

The same problems arose when recording information with pen and paper before computers were ever built. Received on Thu May 11 2006 - 08:51:20 CDT