Re: Principal of view equality?
Date: Mon, 09 Jun 2008 16:53:32 -0300
Evan Keel wrote:
> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>>paul c wrote:
>>>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?
>>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 in database design is kind of like ground in electrical design.
The base relations may not be stored as is. One might derive the value of the base relations from all sorts of different physical structures arranged and distributed all sorts of ways.
One generally achieves physical data independence by deriving the base relations from physical structures. One generally achieves logical data independence by deriving application views from the base relations. One generally describes the database constraints in terms of the base relations.
Regardless, the expression that defines a view constrains the view and nothing else. A constraint declared on a view constrains the entire database. Received on Mon Jun 09 2008 - 21:53:32 CEST