Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Wed, 06 Jul 2005 06:50:09 GMT
Message-ID: <BaLye.138283$vw3.7325787_at_phobos.telenet-ops.be>
VC wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:EtCye.138094$Px7.7310395_at_phobos.telenet-ops.be...
>
>>VC wrote: >> >>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message >>>news:64hye.137293$VX3.7329860_at_phobos.telenet-ops.be... >>> >>>>VC wrote: >>>> >>>>>OK, we want specifically an algebra (RA) primarily for two practical >>>>>reasons. >>>>> >>>>>(A) It's trivial to compose relational algebra operators (create nested >>>>>queries) thanks to the closure property. One can create arbitrarily >>>>>complex queries by using several simple and easily understood relational >>>>>operators (join, projection, etc). I imagine similar freedom is not >>>>>available to the user even with modern network/graph query languages >>>>>(whatever those might be). >>>> >>>>Well, it is available, most of those languages are orthogonal. >>> >>>For example ? One non-relational language example would suffice. >> >>XQuery and OQL spring to mind. Here is a nice overview: >> >>http://citeseer.ist.psu.edu/heuer91principle.html
>
> The article you referenced says:
>
> "As a very important feature of OOQLs we introduced the notion of
> object-preserving
> operator semantics"
>
> My note: apparently to make up for the lack of closure.
> And further:
>
> "That is rather than returning relations [...] we must be able to express
> queries that return
> existing objects. Views [...] are the most evident reason for this
> requirement, called adequacy.
> To our knowledge, none of the OOQL proposals found so far deals
> satisfactorily with this requirement"
>
> And elsewhere in the same article:
>
> "If we have only object generating operations [...] we cannot use the
> constructs like views, updates of views,
> and the equivalence of queries for optimization purposes"
>
> That's essentially the issues with non-relational query languages I
> mentioned in my earlier message.
Yes, but as you can see, these problems have been addressed and not only by them.
>>They also mention a few algebras, by the way.
>
> They do, don't they, but do not provide any example.
>>>>>>>Well, apparently, I am not as smart as most of your students are. >>>>>>>Please provide a definition of "conceptual object type". >>>>>> >>>>>>A unary predicate. >>>>> >>>>>If so, what would it mean to say that object O is of conceptual type T >>>>>given that T is a unary predicate ? >>>> >>>>T(O) holds. >>> >>>What does it mean ? What's the "T(O)" domain of interpratation ? >> >>All the objects in the universe of discourse that is supposed to be >>described by the data model. This set is defined by the modeler, just like >>the exact meaning of the predicate in question.
>
> If the set/universe of discourse consists of all the objects defined by the
> modeller, then there is no point in talking about the type since there is
> only one data type -- the whole universe of discourse.
>>>>>>>Cannot we just use the notion of data type (aka domain) instead ? >>>>>> >>>>>>We can, with the additional restriction that the elements of the >>>>>>domains must have some kind of lexical representation. >>>>> >>>>>Please define "lexical representation". >>>> >>>>A finite string over some finite alphabet. >>> >>>Why do we care about a finite string over some finite alphabet >>>representing a value ? >> >>It ensures that we can show it to the user and the user can enter it via a >>keyboard.
>
> What's that got to do with a logical model ?
- Jan Hidders