Re: insert to projection

From: paul c <>
Date: Sun, 06 Sep 2009 13:43:05 GMT
Message-ID: <J%Oom.43977$PH1.40579_at_edtnps82>

Mr. Scott wrote:
> "paul c" <> wrote in message
> news:uskom.43824$PH1.21867_at_edtnps82...

>> Mr. Scott wrote:
>>> "paul c" <> wrote in message 
>>> news:DGbom.42669$Db2.5159_at_edtnps83...
>>>> Why do implementation languages not allow this?  Surely not for logical 
>>>> reasons?  We can delete from projection because NOT Pa implies NOT Pab, 
>>>> eg., <NOT> R{a} -> <NOT> R{a,b}.  Logically, we can insert to 
>>>> projections because Pab implies Pa.  Isn't the problem really a language 
>>>> deficiency?
>>> I don't understand.  Is the binary predicate P somehow related to the 
>>> unary predicate P, and if so, how exactly?
>> Sure it is, the truth of the tuple <a 1, b 2> implies the truth of the 
>> tuple <a 1> and the falsity of the tuple <a 1> implies the falsity of <a 
>> 1, b 2>, as far as a relation R with predicate P is concerned. Projection 
>> means quantification and vice versa, what's the problem? (Could it be that 
>> predicates aren't recorded?).

> Can you express the relationship formally? Something about your explanation
> doesn't seem right. There can be a row <a 1> in the projection if and only
> if there is a row that is a superset of <a 1> in the table. That works out
> to something like,
> Px iff (exists y exists z Pyz /\ (x = y))
> But this actually denies insert to projections because it is not enough to
> know that there is at least one z, it is necessary to know which z or set of
> z's there are for a given x, unless you want to introduce nulls.

What I wrote could be taken wrong. When I said that "logically" we can insert to a projection it would be have better to say that several projections are inserted when we insert <a 1, b 2, c 3>, eg., R{a} or <a 1> but the converse isn't logical. It's a starting position for figuring out a language definition that would allow insert to projection. I didn't mention rows and tables because I think they are probably not part of a solution. Received on Sun Sep 06 2009 - 15:43:05 CEST

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