Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Alexandr Savinov wrote:
> Jon Heggland schrieb:
> > In article <42b14ba3$1_at_news.fhg.de>, savinov_at_host.com says...
> >>Without queries database is not able to interpret data and to maintain it.
> >
> >
> > Can you elaborate? What interpretation is desirable, what maintenance is
> > impossible?
>
> For example, consider the use of joins. We have several types of them
> including manual joining by means of WHERE. Then in each individual
> query you need to specify all the details of joins. Again, that is
> needed because our database unable to derive necessary information. And
> it is unable to do it because it does not know the semantics of data -
> it simply can retrieve the specified data according to processing
> instructions given in the query.

Are you sure you are familiar with the RM ? One does not specify any "processing instructions" (as one would in a hierarchical or network models) but rather declares a predicate according to which the database produces a desired relation. Access path/optimization/etc are taken care of automatically as no doubt others have already pointed out.

If you mean something different, please elaborate how in your model you avoid the 'join assembly'. Surely, it's not by means of adopting a rigid hierarchical (or some other) structure for your model, or is it ?

> And the semantics is absent because
> there is no acceptable data model.

Would you please skip the word 'semantic' since apparently you do not have a clear definition of it in the context of this discussion ? Alternatively, you could provide some definition, e.g. semantics=='my data organization'(whatever it is) plus 'constraints' (whatever way they are represented in).

>.. Each model then will include as a
> necessary part all relationship and the user needs only specify *what*
> he want to retrieve but not *how* he has to produce the result set.

That's what the RM includes and that's precisely what the user does in it. What problem are we trying to solve here ? I am genuinely curious.

>
> As I mentioned somewhere, a motivating example from UR model might be
> very appropriate for COM as well: we need to compute queries like
>
> SELECT Emploees.name FROM Emploees WHERE Managers.name='Jones' AND
> Products.type'cars'
>
> In terms of MS WinFS it would sound like "I want to get all employees
> with Jones as a manager and related to product with type 'cars'". Note
> that we use 3 tables here (2 for constraints which are propagated and 1
> as a target).
>
> In such an approach the database changes its role. Instead of
> maintaining rows in tables it maintains relationships between data
> items. It is a significant change of paradigm actually.

How so ? Did you miss the 'relationship' part in the E-R model or the foreign key notion ?

[..skipping the semantics part ...]

> The
> position in the hierarchy determines its role (the results of queries
> will depend on it). Detail tables are superconcepts and they are
> positioned all above the master table. In general case all the tables
> have to positioned hierarchically and it is crucial importance for
> modelling.

One would imagine that the hierarchical model, as well as the network one, has been found quite deficient since I do not remember when.

Still, I am curious as to whether you might have some subtle point I am missing. Why won't you provide a side by side comparison case and prove that, yes, your approach (whatever it is) is 'better' than the RM because ...

Simplistic queries like 'get me a car at this price and of the red colour' just do not cut it as the RM handles those quite nicely, thank you very much.

vc
>
> --
> alex
> http://conceptoriented.com
Received on Thu Jun 16 2005 - 18:57:54 CEST