Re: Lucid statement of the MV vs RM position?

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 08 May 2006 02:38:19 GMT
Message-ID: <vay7g.4803$A26.124332@ursa-nb00s0.nbnet.nb.ca>

Marshall Spight wrote:

> Jon Heggland wrote:
>

>>Perhaps I still was unclear; let me try again. Obviously you can
>>postulate an aggregate operator that defined as iterated union, like SUM
>>is iterated addition. Tutorial D does just that, and calls it (perhaps
>>confusingly) UNION. You could call it GROUP instead, but Tutorial D does
>>not. It uses the name GROUP for a unary relation operator that is
>>shorthand for a particular extension/projection; alternatively a
>>summarisation using that iterated union aggregate operator. I honestly
>>don't see why this is so difficult to grasp.

>
> You describe two things. You say they are different, but I don't
> see any differences.
>
> One the one hand, we have "iterated union." On the other hand,
> we have "shorthand for a particular extension/projection", which
> is not very specific, but *could* be a description of what we have
> in the first case.
>
> Can you be more specific about the differences between the
> two?

In the end, I think you and I focused on function while Jon focused on form.

Functionally, GROUP is an aggregate among other things. For instance, it is also a value selector.

Syntactically, Date and Darwen chose to handle it differently than other aggregates. Perhaps to avoid confusion with the UNION aggregate, or perhaps to provide a symmetric balance to UNGROUP. Perhaps for a lot of reasons or perhaps for no particular reason.

One can easily look at GROUP as a convenient shorthand for a combined type conversion and aggregate. Received on Sun May 07 2006 - 21:38:19 CDT