Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Tue, 05 Jul 2005 20:56:04 GMT
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>OK, we want specifically an algebra (RA) primarily for two practical
>>>(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:
They also mention a few algebras, by the way.
>>>>>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 ?
> 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.
>>>>>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.
- Jan Hidders