Re: Lucid statement of the MV vs RM position?

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 05 May 2006 15:24:38 GMT
Message-ID: <W6K6g.126571$P01.21187@pd7tw3no>

Jon Heggland wrote:
> paul c wrote:
>

>>Jon Heggland wrote:
>>
>>>I can't parse the above, but as I read the GROUP definition, R GROUP ()
>>>AS RVA is equivalent to EXTEND R ADD TABLE_DEE AS RVA.
>>>...
>>
>>Oh, maybe this is a question for the authors.  I thought it was
>>equivalent to R JOIN TABLE_DEE 

>
>
> No, R JOIN TABLE_DEE equals R. GROUP always adds an RVA; that's its
> purpose. I checked the GROUP/UNGROUP treatment in "Temporal Data and the
> Relational Model", and there they say explicitly that grouping on no
> attributes is equivalent to extending with TABLE_DEE.
>
> Since GROUP is a unary operator, I don't think it makes sense to talk
> about identity with respect to it (like TABLE_DEE is identity with
> respect to JOIN or whatever the terminology is).
> ...

Thanks, that makes sense as far as ttm is concerned. I often get in trouble with TABLE_DEE, using the shorthand, I suppose one could have written something like R JOIN RELATION {attributenamenotinR, TABLE_DEE}.

    Don't know why I guessed otherwise.

p Received on Fri May 05 2006 - 10:24:38 CDT