Re: insert to projection

From: paul c <>
Date: Fri, 04 Sep 2009 20:55:22 GMT
Message-ID: <_8fom.42729$Db2.40393_at_edtnps83>

Tegiri Nenashi wrote:
> Let's get back to the main point. I'm suggesting that analysis of
> simple relational expressions update won't be useful. Suppose a view
> is composition of join, selection, and projection. Each of these can
> violate some restrictions that you would propose based on analysis of
> basic relation updates. Yet, the view can still be updatable.

That's more like it. Now it's a matter of either using the recording form available or if that is thought deficient, coming up with a different recording form. Even if for some arbitrary reason one doesn't want to update views, it remains instructive to construct an equivalent set of bases and ask what results those would produce. Then ask why the view should be any different. From an RA perspective a view "expression" is no different than a base constraint, such as a key constraint or any other one could dream up. In fact, a base "update" to a constrained table arbitrarily (and conventionally) "chooses" to replace only the value of one of the several relations involved. If a language is based on a convention that says only named expresssions can have their values replaced, I wouldn't argue with that. Maybe part of the problem is that many people think an implementation language implements relational algebra, when all it needs to do is not violate RA, eg., by introducing ambiguity. Received on Fri Sep 04 2009 - 22:55:22 CEST

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