Re: constraints in algebra instead of calculus
From: Brian Selzer <brian_at_selzer-software.com>
Date: Wed, 23 May 2007 16:26:55 GMT
Message-ID: <jXZ4i.9350$RX.3568_at_newssvr11.news.prodigy.net>
>> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> news:nnM4i.206459$DE1.56863_at_pd7urf2no...
>>
>>>Brian Selzer wrote:
>>>
>>>>...
>>>>V is not in BCNF: since StaffId --> CourseId in Teaches, StaffId -->
>>>>CourseId in V.
>>>>...
>>>
>>>I'll think about your other points, it could be that I misunderstood his
>>>point - meanwhile, I must say I'm not too concerned about V's 'form' as I
>>>think he used it just to make his exposition clearer - in a suitable
>>>language, I would guess, one would not be forced to define a view in
>>>order to express the constraint.
>>>
>>
>>
>> Sometimes it makes them more succinct. There is danger, however. I
>> don't think it's decidable how a constraint defined on an arbitrary view
>> affects the closure of the set of constraints on the database schema.
>> Can a constraint defined on a view always be a logical consequence of a
>> set of constraints defined on the underlying base relations? I don't
>> think so.
>> ...
Date: Wed, 23 May 2007 16:26:55 GMT
Message-ID: <jXZ4i.9350$RX.3568_at_newssvr11.news.prodigy.net>
"paul c" <toledobythesea_at_oohay.ac> wrote in message news:ubY4i.212893$aG1.147055_at_pd7urf3no...
> Brian Selzer wrote:
>> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> news:nnM4i.206459$DE1.56863_at_pd7urf2no...
>>
>>>Brian Selzer wrote:
>>>
>>>>...
>>>>V is not in BCNF: since StaffId --> CourseId in Teaches, StaffId -->
>>>>CourseId in V.
>>>>...
>>>
>>>I'll think about your other points, it could be that I misunderstood his
>>>point - meanwhile, I must say I'm not too concerned about V's 'form' as I
>>>think he used it just to make his exposition clearer - in a suitable
>>>language, I would guess, one would not be forced to define a view in
>>>order to express the constraint.
>>>
>>
>>
>> Sometimes it makes them more succinct. There is danger, however. I
>> don't think it's decidable how a constraint defined on an arbitrary view
>> affects the closure of the set of constraints on the database schema.
>> Can a constraint defined on a view always be a logical consequence of a
>> set of constraints defined on the underlying base relations? I don't
>> think so.
>> ...
> > If view V = A & B, surely constraint C on the view, giving V & C, would be > manipulated by an engine with the same effect as the expression (A & C) & > (B & C). > > I've never heard of anybody requiring expressions to result in BCNF > values, surely that would cripple an algebra. >
They don't. Nor should they. But an expression defining a view can involve anything, restriction, difference, aggregation, division. How does a constraint on the view map to constraints on the base tables?
> p Received on Wed May 23 2007 - 18:26:55 CEST