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Home -> Community -> Usenet -> comp.databases.theory -> Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Alexandr Savinov wrote:
> VC schrieb:
> > "Alexandr Savinov" <savinov_at_host.com> wrote in message
> >>Yes, we need to add more information into our model so that the database
> >>knows what to do if queries do not have enough information. In other
> >>words, the model has more information while queries are simpler.
> >>
> >
> >
> > Please explain what exactly you mean by the expression "the database knows
> > what to do if the queries do not have enough information". 'Knows' in what
> > sense ? As an AI specimen or in some other sense ? Also please give some
> > specific examples of those queries illustrating your statement.
>
> I feel that even if I answer you will still be unsatisfied. Here is one
> posssible concrete answer. My database needs to be able to answer the
> question: "retrieve(Employees) where Manager='Jones'. For this query to
> execute the database has to know more about relationships between data
> items and data semantics.
I hope you know that in a relational database both the query formulation and the answer are trivial.
>In particular, the data is NOT a collection of
> tables - it is hierarchically and multidimensionally ordered tables (I
> write in terms of RM).
You cannot be possibly writing "in terms of RM" because what you are describing ain't relational ("hierarchically and multidimensionally ordered tables").
> Do you feel a difference between a flat
> collection and a lattice?
It depends on what you mean by a "flat collection" and "a lattice". Whilst I can infer that by a "flat collection" you might mean a set of relvars, I cannot figure out what you mean by "a lattice" (a discrete subgroup of a finite-dimensional vector space, or a partially ordered set with certain properties, or something entirely different).
>Or I should explain it some "more specific way"?
Please do before we investigate a more complex issue of the database "knowing".
> >>[...] Semantics can be defined as both constraints with data or only data.
> >
> >
> > No, it cannot. In your private vocabulary maybe.
>
> If you look at different papers then you can easily find different
> definitions and/or interpretations.
One can loosely/informally say that "database semantics" for the relational model is the databases RVs (not some vague "data") *and* constraints.
> Semantics just like syntax, data or
> program is a kind of term that is overloaded and needs to be defined
> concretely for each new theory or its variation. Or you have an ultimate
> and final definition of the term "semantics"?
I can give you one for FOL: a formula F semantics is: the interpretation I satisfies the formula F in the model M. In the relational model, substitute the word 'query' for 'formula' and you'll get the query semantics.
If a word is so vague, in a given context, as to be devoid of clear meaning, why use it ? Just state what you mean clearly and unambiguously.
>
> >>function applied to a set. It is more general - strictly speaking we can
> >>aggregate (project) everything and deproject everything.
> >
> >
> > What's 'project' and 'deproject' supposed to mean ?
>
> Sorry, but I am not able to describe it formally in the format if forum
> for obvious reasons. Informally, if you have hierarchical dimensions
> then you can propagate avialable information (data items) or
> constraints upward or downward.
It appears you are talking here about a multidimensional model and its constraint implementation. Am I right ? If not, how do you 'propagate constraints' in the relational model ? Could you give a for-example ?
> > Also, you still did not answer how the notion of 'singularity' and
> > 'delta-function' is related to nulls. Eagerly awaiting.
>
> As far as I remember I explained that. Here is that definition again:
> - a value (a variable taking a value) = a possiblity distribution which
> is equal 0 (impossible) everywhere except for one point.
I am confused. How a value can be a function ? A value of a certain type, say integer, is a member of the set of integers. Now, as you know, a function is a a mapping between sets. So how can you say that a value is a function ? Are you using a theory where the function is a more primitive notion than an element of a set ? Please explain.
[...skipped...]
> The term delta-function and singularity are used to denote a function
> that is 0 everywhere except for one point (and the integral is 1 if you
> like).
Firstly, a singularity is not a function at all, but an element(s) of the function domain where the function is undefined. Secondly, technically speaking, delta-function is not a function either.
It is true that the delta function can be thought of as a probability distribution although I still do not see what additional insight the probability notion adds to the concept of null 'values'.
>But it is just for analogy for those who understands what it is.
Indeed.
vc
>
> --
> alex
> http://conceptoriented.com
Received on Thu Jun 16 2005 - 11:18:21 CDT