Re: <OR> predicate?

On 29/09/2010 7:40 AM, Vadim Tropashko wrote:
> "We are able to dispense with restrict (WHERE), EXTEND, and
> SUMMARIZE, since these operators all become further special
> cases of<AND> . Note: EXTEND and SUMMARIZE were not included
> in Codd's original algebra but were added subsequently [132]."
>
> This argument is not convincing. Later on there is more detailed
> treatment:
>
> "Dispensing with restrict (WHERE), EXTEND, and SUMMARIZE
> Restrict (WHERE), EXTEND, and SUMMARIZE all require certain
> operators to be invoked as part of their execution. In the case of
> restrict, the operators in question return values (truth values, to
> be precise) that are used to disqualify certain tuples from
> appearing in the result relation; ...
> It occurred to us that it made sense, and could possibly be
> useful, to treat such operators as relations.

Just a side comment, not to detract from the more important question about summarize: it could be that I have a mental block about this but regarding restrict, it's defined on the <AND> and <NOT> operators which return relations, not truth values per se. The symbol for set membership is used in the definitions of those ops which I'd say gives a truth value but I don't think it's right to let the terms in the definitions percolate to the result of the operators being defined (if that's what D&D mean here). (and assuming that the 'operators' talked about are just the A-algebra ops, the ones that are 'executed' by a machine, if only a theoretical one.) Received on Wed Sep 29 2010 - 17:18:48 CEST