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

Date: Thu, 16 Jun 2005 18:47:18 +0200

Message-ID: <42b1ad17$1_at_news.fhg.de>

vc schrieb:

> 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.*

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*>*

>>In particular, the data is NOT a collection of >>tables - it is hierarchically and multidimensionally ordered tables (I >>write in terms of RM).

*>*

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> 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"?

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> 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

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.

*>*

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> 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.

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> 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.

>>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'.*

-- http://conceptoriented.comReceived on Thu Jun 16 2005 - 18:47:18 CEST