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From: David BL <davidbl@iinet.net.au>
Newsgroups: comp.databases.theory
Subject: Re: relative complement?
Date: Mon, 4 Apr 2011 01:54:44 -0700 (PDT)
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On Mar 19, 7:47 am, paul c <anonym...@not-for-mail.invalid> wrote:

> Given that preference, I think join deletes and union inserts become
> mechanical problems.  For me, in the end it amounts simply to a user
> stating that the tuples of the literal B relation are (all) true and
> that B's predicate is immaterial.  With those two assertions, the
> A-algebra is sufficient to describe what results a language should
> produce without ambiguity.

I accept the idea that update operators on views can be defined to be
unambiguous, dispelling a common argument against view updates such as
join deletes and union inserts.

However, I'm very suspicious of an application allowing users to
update a non-injective view of the data.   More specifically if users
of an application cannot in general distinguish distinct values of
some base relvar how can they reasonably be expected to have
permission to update that relvar?