Re: Minutes for the 2010-10-21 RDB2RDF meeting + semantics discussion

On Thu, 2010-10-21 at 16:32 -0400, Alexandre Bertails wrote: 
> Hello guys,
> here are the minutes for "RDB2RDF - Formal mapping - semantics" staged
> at [1]. Sorry for all the @@ but I had troubles to associate the voices
> with the right people.
> == Quick Summary ==
> Here is a quick summary:
> * importance of the 7 use-cases which should have their own section
> * it's ok to have some examples covering several use-cases (if it's
> said)
> * consensus around the SQL terminology instead of the Relational Algebra
> one, because of the intended public

Not this public.  I find the relational model and terminology much
easier to understand.  It is also strongly typed (by default), so you
cannot add or compare AGE and WEIGHT columns just because they are both
numeric (unless of course you really need to, and take the trouble to
define the required operators).  This makes it safer to reason with.

> * Marcelo and ?? proposed to update EricP's documents based on the
> previous points. Should be done next week.
> * discussion about what "semantics" means in the context of RDB2RDF, no
> consensus. See below for more information.
> * to answer the previous point, Marcello will send an email with the
> right informations (a digitalized book) and will give some context
> == Formal Mapping ==
> I did not scribe while I was speaking, at the end of the telcon. Here
> are my notes about *what* is the mapping:
> 1. simple case: it's only a Default Mapping, so it's a function
> [ RDB2RDF : RDB → RDF ]
> 2. difficult case: it's a function interpreting a rule language (R2ML),
> so it's a function [ RDB2RDF : (RDB×R2ML) → RDF ]. In this context, the
> Default Mapping is just a particular case where you use the empty value
> as an inhabitant for R2ML.
> In both cases, you need to start with the simple case, the Default
> Mapping, then you can build something more complicated on top of it.
> == Semantics ==
> I'm looking forward for Marcello's document about his definition. And I
> hope he will also comment what some others think.
> [[
> <PatH> We could (not on IRC) draw this as a 'square' of functors which
> we want to commute.
> ]]
> Here is it is, in the case of the Default Mapping.
> Typed multi-sets   -- ?? -→   Set of triples
>       ↑                            ↑
> RDB semantics                 RDF semantics
>       |                            |
>      RDB       -- mapping -→      RDF

Is there really much "semantics" in RDB?  A database is just a
collection of tuples of typed values, and SQL DML provides different
ways to compose further collections of tuples.  All useful semantics of
RDB are imposed from outside the system.  The relational model does not
provide for enough meaningful variation among tokens to have anything
more than a primitive semantics: only type, value, tuple-hood, and
relation provenance.

Because relational tables have no useful semantics, no mapping to a
semantically richer system is possible.  It is possible, however, to
make inferences about your domain ontology from the existence of tuples
in RDB relations or views.  In order to do this you must add axioms or
rules.  It is not a mapping problem, although in degenerate cases it
might appear so.  What you are calling the "direct mapping" is more like
a language binding, a way of expressing relational concepts and
constructs in RDF idiom.  Perhaps there are lessons from the XML binding
defined by SQL-2008:14. (I have not looked at this spec, so I do not
know what it contains--but standard XML vocabulary for the SQL domain is
long overdue.)


Received on Saturday, 23 October 2010 14:56:48 UTC