Re: Principal of view equality?
Date: Wed, 4 Jun 2008 00:35:48 -0400
Message-ID: <FYo1k.4588$jI5.1548_at_flpi148.ffdc.sbc.com>
"paul c" <toledobysea_at_ac.ooyah> wrote in message
news:Tsn1k.4999$C12.2720_at_pd7urf3no...
>I was just looking at Codd's RM 2 book again (the rather short chapter on
>views from acm.org) and it seemed to me that what he wrote took it as
>essential that a view must always equal the expression that defines the
>view. If so, does this in effect constitute a kind of indirect constraint
>on any base relations involved?
>
If you issue a query, does this in effect constitute a kind of indirect constraint on any base relations involved?
From P --> Q and ~P, you can't conclude ~Q, but from P --> Q and ~Q, you can conclude ~P. This is applicable here because that something appears in a base relation doesn't necessarily mean that it will appear in a view derived from the base relation, so from the absence of that something in the view you can't conclude that it doesn't appear in the base relation. On the other hand, from the absence of that something in the base relations, you should be able to conclude that it shouldn't appear in the view. Received on Wed Jun 04 2008 - 06:35:48 CEST