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RE: relational mapping?

From: Danny Ayers <danny@panlanka.net>
Date: Fri, 4 May 2001 21:27:57 +0600
To: "Cayzer, Steve" <Steve_Cayzer@hplb.hpl.hp.com>
Cc: "RDF-Interest" <www-rdf-interest@w3.org>
Message-ID: <EBEPLGMHCDOJJJPCFHEFEEOODEAA.danny@panlanka.net>
Thanks, this certainly looks like a way in - and you're right, the
terminology does sound straight out of DAML.
I've opted for a lazier approach to the problem (RDBMS -> JDBC -> Objects ->
RDF-XML), but as I'm trying to learn a bit of logic I'll still be taking a
look. ERL? E-R logic maybe?

As far as the logic & RDBMS stuff is concerned, there does seem to be a
whole load of potential - pretty much anything you like (1st order anyway, I
guess) as long as you use the relational model as your starting point rather
than a DB vendor's wizard. I was sceptical of some of the criticism of SQL &
commercial RDBMS by the relational purists, until I came across 'Writing
Complex SQL Queries that Require Universal Quantifiers' [1] (SQL has EXISTS,
but it still gets messy). Now I agree - SQL is a hack. Even though the
syntax of XQuery [2] brought back memories of coding for a Commodore PET, at
least the designers appear (to this untrained eye) to have addressed the
fundamentals.

[1] http://www.cpsc.ucalgary.ca/~kawash
[2] http://www.w3.org/TR/xquery

---
Danny Ayers
http://www.isacat.net

<- -----Original Message-----
<- From: www-rdf-interest-request@w3.org
<- [mailto:www-rdf-interest-request@w3.org]On Behalf Of Cayzer, Steve
<- Sent: 04 May 2001 14:33
<- To: 'Danny Ayers'
<- Cc: RDF-Interest
<- Subject: RE: relational mapping?
<-
<-
<- fyi, I came across a paper recently which describes a bijective
<- transformation
<- from an Entity Relationship model to ERL (a simplified description logic)
<-
<- The authors' intention was (I think) more to provide data
<- validation above
<- and beyond what is possible
<- in a typical RDB schema but it seems to me you could map ERL to RDF (via
<- DAML?)
<- [I confess I merely skimmed it but it looks like it could be
<- useful to you]
<-
<- Author Hacid, M.-S.,Petit, J.-M.,Tourmani, F.
<- Journal Knowledge and Information Systems
<- Volume vol.3, no.1
<- Abstract We address the problem of reasoning about database conceptual
<- schemas by exploiting the possibility of using a description logic. We
<- develop an approach by using as a foundation an entity-relationship model
<- that displays features such as ISA, disjointness and cardinality
<- constraints. We propose an equivalence-preserving transformation
<- of entity
<- relationship schemas into terminologies in a description logic. This
<- equivalence, based on the measure of information capacity,
<- ensures that the
<- semantics of entity-relationship schemas is accurately captured by the
<- corresponding terminologies. As a consequence, reasoning on
<- entity-relationship schemas is appropriately reduced to reasoning on
<- terminologies in a description logic
<- InclusivePages 52-80
<- CorporateSource LISI, Villeurbanne, France
<- Publisher Springer-Verlag
<- DateOfPublication Feb. 2001
<-
<- hth
<-
<- Steve
<-
<- [btw, I am not cross-posting this to cut down on RDF Logic
<- traffic. But if
<- any logicians are reading this it would be
<- interesting to see what they make of ERL]
<-
<- ________________________________________________________________________
<- Steve Cayzer    HP Labs, Bristol, UK    mailto:Steve_Cayzer@hp.com
<-
<-
<- -----Original Message-----
<- From: Danny Ayers [mailto:danny@panlanka.net]
<- Sent: 03 May 2001 17:59
<- To: Jan Grant
<- Cc: Www-Rdf-Logic; RDF-Interest
<- Subject: RE: relational mapping?
<-
<-
<- <- I've done quite a bit of thinking about it; got some paper notes which
<- <- I'm in the (slow) process of typing up. You're right, there's a simple
<- <- mechanical mapping of rows in a table to RDF; what you lose by this is
<- <- the natural linking of properties.
<-
<- Not sure I understand - linking of properties in the RDF?
<-
<- <- For a sufficiently normalised relational schema*, you can generally
<- <- produce a mapping
<- <- 	(primary key) -> resource
<- <- 	(other values) -> properties
<- <- 	(foreign key) -> link to resource representing primary key for
<- <- 			foreign table
<-
<- Great - this looks very promising, just the kind of thing I was
<- looking for,
<- I'm going to have to get pencil & paper.
<-
<- <- * that is, very (what, fifth NF?): for instance, moving
<- one-to-one data
<- <- into a separate table if the data describes a separate concept; it's
<- <- generally possible to produce a normalised schema from a less
<- normalised
<- <- (ie, more real-world) one with the judicious use of views.
<-
<- Ugh! I've been postponing re-reading on the later NFs, hoping I wouldn't
<- have to. Rats.
<-
<- I foolishly didn't say what I was wanting to do with the mapping
<- - off list
<- I've been mailed some good links for going RDF -> RDBMS
<- (thanks!), but I'm
<- really looking at RDBMS -> RDF and (as you seem to have mind-read) was
<- wondering whether there was a way up from relational <-> RDF model.
<-
<- The plan is to expose a RDBMS, starting with metadata, trying to
<- preserve as
<- much of the logical structure as possible. I'm currently
<- thinking this way :
<- pump out the top-level information as RDF, linked down to a dynamically
<- generated XML Schema that will pass on the constraints for any
<- actual data
<- requested, which will in turn be presented as straight XML. I reckon if I
<- can get the RDF right, the rest should follow mechanically. Hopefully ;-)
<-
<- I was considering doing the mapping within the DB, generating a table of
<- triple using the logical capabilities of the RDBMS. This is very
<- appealing
<- from the point of view of integrity (& probably efficiency), but
<- the stuff
<- will still need to be poured out in a form that other systems will
<- understand, and application-level constraints will almost certainly be
<- needed at the other end anyway, so it's code-hacking time
<- whatever happens.
<-
<- Thanks for the info. Now where's that pencil...
<-
Received on Friday, 4 May 2001 11:33:54 GMT

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