# Re: insert to projection

From: paul c <toledobythesea_at_oohay.ac>

Date: Sun, 06 Sep 2009 13:43:05 GMT

Message-ID: <J%Oom.43977$PH1.40579_at_edtnps82>

> Can you express the relationship formally? Something about your explanation

Date: Sun, 06 Sep 2009 13:43:05 GMT

Message-ID: <J%Oom.43977$PH1.40579_at_edtnps82>

Mr. Scott wrote:

> "paul c" <toledobythesea_at_oohay.ac> wrote in message

> news:uskom.43824$PH1.21867_at_edtnps82...

>> Mr. Scott wrote: >>> "paul c" <toledobythesea_at_oohay.ac> 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