Date: Thu, 16 Jun 2005 18:47:18 +0200
From: Alexandr Savinov
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Subject: Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
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vc schrieb:
> Alexandr Savinov wrote:
>
>>VC schrieb:
>>
>>>"Alexandr Savinov" 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.
So you recognize that one and the same term may have different
definitions? For example, lingusts may have their own definition.
> 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.
In COM:
syntax = a set of concepts
semantics = a set of items
where
concept = a combination of superconcepts
item = a combination of superitems
and so on (please, do not ask me to continue because I am not able to do
it here in the forum). But I am afraid the problem is not in having
clear and unambiguous definitions.
>>>>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.
I already mentioned that terms, especially general ones, may change
their meaning. The terms value may be defined differently. In
particular, it can be defined via membership *function*. The term
function can be applied to such a strange somewhat illegal construct as
delta-function (which is not a function as you correctly noticed) and so
on.
> [...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'.
It seems to me that we are talking about everything and nothing
simultaniously. You vary the focus of the discussion arbitrarily
hierarchically (between very general and very specific level) and
multidimensionally (changing the direction within one level on 90
degrees). So I am lost and do not understand where we are now.
--
http://conceptoriented.com