Re: Relation Schemata vs. Relation Variables
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 06 Sep 2006 14:06:42 GMT
Message-ID: <SBALg.9957$9u.139979_at_ursa-nb00s0.nbnet.nb.ca>
>>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>>news:1157532864.768886.10750_at_d34g2000cwd.googlegroups.com...
>>
>>>Brian Selzer wrote:
>>>
>>>>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>>>>news:1157457516.222077.154380_at_b28g2000cwb.googlegroups.com...
>>>>
>>>>>Sets of facts can and do change, and transitional constraints restrict
>>>>>wich transitions from one set of fact to another are allowed. I don't
>>>>>see a fundamental problem here. Note btw. that they are a strict
>>>>>subclass of the restrictions that might be expressed by some kind of
>>>>>temporal logic.
>>>>
>>>>I don't understand what you mean. Are you saying that transition
>>>>constraints can be expressed as state constraints?
>>>
>>>A transitional constraint is a binary predicate over states. One
>>>argument is the old state and the other the new state. Or, put in
>>>another way, a transition constraint constrains the transitions. This,
>>>I would say, is pretty much the definition of the term.
>>>
>>>Or did I misunderstand your question and are you asking about temporal
>>>logics?
>>>
>>
>>No. I just wanted to be sure that we're on the same page.
>>
>>The point that I was making in the original post is that because keys can
>>change, there isn't enough information given only the old state and the new
>>state to pair up the values in the old state with those in the new state for
>>comparison.
Date: Wed, 06 Sep 2006 14:06:42 GMT
Message-ID: <SBALg.9957$9u.139979_at_ursa-nb00s0.nbnet.nb.ca>
JOG wrote:
> Brian Selzer wrote: >
>>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>>news:1157532864.768886.10750_at_d34g2000cwd.googlegroups.com...
>>
>>>Brian Selzer wrote:
>>>
>>>>"Jan Hidders" <hidders_at_gmail.com> wrote in message
>>>>news:1157457516.222077.154380_at_b28g2000cwb.googlegroups.com...
>>>>
>>>>>Sets of facts can and do change, and transitional constraints restrict
>>>>>wich transitions from one set of fact to another are allowed. I don't
>>>>>see a fundamental problem here. Note btw. that they are a strict
>>>>>subclass of the restrictions that might be expressed by some kind of
>>>>>temporal logic.
>>>>
>>>>I don't understand what you mean. Are you saying that transition
>>>>constraints can be expressed as state constraints?
>>>
>>>A transitional constraint is a binary predicate over states. One
>>>argument is the old state and the other the new state. Or, put in
>>>another way, a transition constraint constrains the transitions. This,
>>>I would say, is pretty much the definition of the term.
>>>
>>>Or did I misunderstand your question and are you asking about temporal
>>>logics?
>>>
>>
>>No. I just wanted to be sure that we're on the same page.
>>
>>The point that I was making in the original post is that because keys can
>>change, there isn't enough information given only the old state and the new
>>state to pair up the values in the old state with those in the new state for
>>comparison.
> > You cannot pair up values David. You can only compare the sets as a > whole.
I think you misspoke. I draw your attention yet again to Date's _Principle of Incoherence_: "It is very difficult to respond coherently to that which is incoherent."
One can pair up values any number of ways: least, greatest, lesser, greater, least greater, greatest lesser etc.
Cartesian product and restrict have the effect that one can pair up tuples within relations ie. elements within sets. Received on Wed Sep 06 2006 - 16:06:42 CEST