Re: Principal of view equality?
Date: Tue, 10 Jun 2008 13:21:51 -0400
"Evan Keel" <evankeel_at_sbcglobal.net> wrote in message
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> "Evan Keel" <evankeel_at_sbcglobal.net> wrote in message
>> > "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>> > news:4846bc21$0$4072$9a566e8b_at_news.aliant.net...
>> >> paul c wrote:
>> >> > I was just looking at Codd's RM 2 book again (the rather short
>> >> > 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
>> >> > view. If so, does this in effect constitute a kind of indirect
>> >> > constraint on any base relations involved?
>> >> No. It does, however, specify constraints on the value of the view.
>> >> Tuples can exist in the base relations that have no effect on the
>> >> value
>> >> of the view.
>> > I thought that "base relations" were also views. What am I missing?
>> A base relation is not a derived relation.
> Yes, it is.
That's news to me. According to Codd (pages 17-18 of his 1990 book):
Those relations, or R-tables, that are internally represented by stored data in some implementation-defined way are called the /base relations/ or /base R-tables/. All R-tables other than base R-tables are called /derived relations/ or, synonymously, /derived R-tables/. An example of a derived relation is a /view/. A view is a virtual R-table defined in terms of other R-tables, and is represented by its defining expression only. Received on Tue Jun 10 2008 - 19:21:51 CEST