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Re: RDB2RDF WG agenda for 2010-10-19 meeting 1600 UTC

From: Alexandre Bertails <bertails@w3.org>
Date: Sun, 17 Oct 2010 23:45:11 -0400
To: Paul Tyson <phtyson@sbcglobal.net>
Cc: ashok.malhotra@oracle.com, Eric Prud'hommeaux <eric@w3.org>, Michael Hausenblas <michael.hausenblas@deri.org>, RDB2RDF WG <public-rdb2rdf-wg@w3.org>
Message-ID: <1287373511.15305.31.camel@simplet>
Hi Paul,

my comment are in line.

On Sun, 2010-10-17 at 16:36 -0500, Paul Tyson wrote:
> On Sun, 2010-10-17 at 14:24 -0400, Alexandre Bertails wrote:
> > On Fri, 2010-10-15 at 15:57 -0700, ashok malhotra wrote:
> > > I will not be able to make the call on Tuesday.
> > > Can we agree that Eric's description is accurate and correct?
> > 
> > Eric, I like your document. Giving a concrete syntax for the input is a
> > nice thing to do to make it explicit so that any RDBMS user can
> > understand what your document is about. By the way, may I suggest you to
> > precise that you need only two subsets of SQL:
> > * the Data Definition Language to describe your database
> > * the Data Manipulation Language that gives you the INSERT
> > It's important as you don't need any query at that point.
> > 
> > As nobody defines mapping in terms of concrete syntax, I guess we will
> > need some sort of abstraction (ie. AST) to reason about, one for the
> > input (SQL Data Definition Language + Data Manipulation Language /
> > INSERT) and one for the output (RDF). The latter is easy as we already
> > have [1] as a W3C Recommendation. Maybe you already have something in
> > mind for a SQL AST but here is an example of what one can expect from
> > your first example:
> > 
> > [[
> > CREATE TABLE Addresses (ID INT, city CHAR(10), state CHAR(2), PRIMARY
> > KEY(ID))
> > CREATE TABLE People (ID INT, fname CHAR(10), addr INT, PRIMARY KEY(ID),
> > FOREIGN KEY(addr) REFERENCES Addresses(ID))
> > INSERT INTO Addresses (ID, city, state) VALUES (18, "Cambridge", "MA")
> > INSERT INTO People (ID, fname, addr) VALUES (7, "Bob", 18)
> > INSERT INTO People (ID, fname, addr) VALUES (8, "Sue", NULL)
> > ]]
> > 
> > would give something like that: (for better indentation/understanding,
> > please see the attached file)
> > 
> > [[
> > database(relation(name("Addresses"),
> >                   header(attribute("ID") → type(int)×PrimaryKey,
> >                          attribute("city") → type(char),
> >                          attribute("state") → type(char)),
> >                   data(tuple("ID" → 18, "city" → "Cambridge", "state" →
> > "MA")))
> >          relation(name("People"),
> >                   header(attribute("ID") → type(int)×PrimaryKey,
> >                          attribute("fname") → type(char),
> >                          attribute("addr") →
> > type(int)×ForeignKey("Adresses", "ID")),
> >                   data(tuple("ID" → 7, "fname" → "bob", "addr" → 18),
> >                        tuple("ID" → 8, "fname" → "sue", "addr" →
> > null))))
> > ]]
> 
> Or why not use an RDF schema of the relational model?  See attached
> rdb-schema.ttl for one concept of the schema, and rdb-ex.ttl for the
> database examples.  The schema is based on the simplest form of
> relational model, which would probably support most use cases.  It could
> as well be put into the terminology of SQL-2008 using SQL-schema,
> SQL-table, -column, -row, etc.

I can understand your proposition to use RDF to describe the database in
two different ways:
1. your RDF *is already* the result of the direct mapping. Even if it's
only a raw description, it's already some RDF from taken from the
description of the database and the data inside. Then, you want to use
some other Semantic Web technologies to modify this RDF. And I agree, it
does the job.
2. this is only a description in RDF for the input. In that case, it's
only a serialization among others, and it's not suitable to reason
about. Think about the mapping itself: by definition [1], it's a
function from RDB to RDF. To define such a function, you need some
abstraction.

We have to use the right tools for the right job. So the question this
WG has to answer first is: are we using the right tools right now?
Again, let's think about what a mapping actually is.

The RDB2RDF mapping is a *function* from RDB (the input) to RDF (the
output).

A good candidate for an RDB concrete syntax is the common SQL language
(description + data manipulation) as it's already out there and widely
known. Maybe there is already an AST for that, *widely accepted* by the
community, the researchers or written in a document as a standard. I
don't know. If it's not the case, we have to decide which features we
want and write our own AST that we *can actually manipulate* in the
mapping/function.

We are lucky as the RDF abstract syntax is *already* defined in [2]. The
AST is explicitly given in plain English.

> I hesitate to include constraints in this schema.  They would be better
> expressed in RIF.

My feeling is I also want to see these constraints in the output. It
would be a shame to throw them away.

> > Actually, this little example raises some questions. See below.
> > 
> > Does R2ML want to map the Data Definition Language to some generated
> > ontology?
> 
> I share Alexandre's confusion on this point.  The example doesn't define
> a good target ontology.
> 
> I have only recently started following the group again.  At the end of
> the XG I thought the direction was to use RIF to handle the mapping into
> a well-defined domain ontology.  I couldn't find in the archives any
> discussion about why RIF was not suitable.  Does someone have a pointer
> to the relevant discussions?
> 
> > 
> > If we take SQL types into account, do we want to know the constraints?
> > That means: is type(char) enough or do I need type(char(10))?
> 
> It would be good to have a standard mechanism to refer to all the
> standard SQL datatypes and datatype families (e.g., char(n)), perhaps
> using OWL datatype definition facilities.

+1

> > Also, one can assume that the built AST comes from valid SQL CREATE
> > TABLE / INSERT. But I think it would be safer to make some assumptions
> > explicit, like for example: "for any tuple, for any (attribute → value)
> > within this tuple, the declared type in the header and the actual type
> > for the value MUST be the same". If it's not the case, well, the mapping
> > does not make any sense :-) One can implement that as a light type
> > system on top of the AST, or we can also decide to make it part of the
> > mapping.
> 
> Again I would ask why not use RIF for expressing these constraints.

+1, but only for the output.

Alexandre.

[1] http://en.wikipedia.org/wiki/Map_%28mathematics%29
[2]
http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Graph-syntax


> 
> Regards,
> --Paul
> 
> > 
> > > The semantics can then be expressed in Datalog.
> > 
> > I was wondering what people call "semantics" for such a mapping and what
> > can kind of statements they expect in the case of R2ML?
> > 
> > I was taught that datalog was just a subset of Prolog, used to define
> > new relations from other relations using deductive logic. Here, we just
> > want to go from one model (RDB) to another one (RDF).
> > 
> > Alexandre.
> > 
> > > All the best, Ashok
> > > 
> > > On 10/15/2010 12:48 PM, Eric Prud'hommeaux wrote:
> > > > * Michael Hausenblas<michael.hausenblas@deri.org>  [2010-10-15 13:44+0100]
> > > >> All,
> > > >>
> > > >> Below the agenda for our next week's meeting. We now focus on addressing the
> > > >> remaining issues (such as document structure, etc.) and the mapping
> > > >> semantics in a high-level, non-formal way. The goal is to publish the FPWD
> > > >> next week.
> > > > I'd also like to get some feedback on whether
> > > >    http://www.w3.org/2010/10/12-Direct-Tests
> > > > match the WG's expectations of what the generated graphs would look like.
> > > >
> > > >> Cheers,
> > > >>        Michael
> > > >>
> > > >> ---------------------------------------------------------
> > > >> AGENDA Teleconference
> > > >> W3C RDB2RDF Working Group telephone conference 2010-10-19
> > > >> ----------------------------------------------------------
> > > >> Tuesday, 19 October *16:00-17:00 UTC* Local time:
> > > >> http://www.timeanddate.com/worldclock/fixedtime.html?month=10&day=19&year=20
> > > >> 10&hour=16&min=00&sec=0
> > > >> Bridge US: +1-617-761-6200 (Zakim) Conference code : 7322733# (spells
> > > >> "RDB2RDF")
> > > >> Duration : 60 minutes
> > > >> -------------------------------------------------------------------
> > > >> IRC channel : #RDB2RDF on irc.w3.org:6665 W3C IRC Web Client :
> > > >> http://www.w3.org/2001/01/cgi-irc
> > > >> Zakim information : http://www.w3.org/2002/01/UsingZakim
> > > >> Zakim bridge monitor : http://www.w3.org/1998/12/bridge/Zakim.html
> > > >> Zakim IRC bot : http://www.w3.org/2001/12/zakim-irc-bot.html
> > > >> -------------------------------------------------------------------
> > > >>
> > > >> Chair: Michael
> > > >> Scribe: Zakim, pick a victim
> > > >>
> > > >> 1. Admin
> > > >> PROPOSAL: Accept the minutes of last meeting, see
> > > >> http://www.w3.org/2010/10/12-rdb2rdf-minutes.html
> > > >>
> > > >> Review open actions, see
> > > >> http://www.w3.org/2001/sw/rdb2rdf/track/actions/open
> > > >>
> > > >> 2. FPWD "Relational Database to RDF Mapping Language"
> > > >> http://www.w3.org/2001/sw/rdb2rdf/r2rml/
> > > >>
> > > >> Comments see following threads:
> > > >> http://lists.w3.org/Archives/Public/public-rdb2rdf-wg/2010Oct/0028.html
> > > >>
> > > >> 3. AOB
> > > >>
> > > >> Cheers,
> > > >>        Michael
> > > >>
> > > >> -- 
> > > >> Dr. Michael Hausenblas
> > > >> LiDRC - Linked Data Research Centre
> > > >> DERI - Digital Enterprise Research Institute
> > > >> NUIG - National University of Ireland, Galway
> > > >> Ireland, Europe
> > > >> Tel. +353 91 495730
> > > >> http://linkeddata.deri.ie/
> > > >> http://sw-app.org/about.html
> > > >>
> > > >>
> > > >>
> > > 
> > > 
> > 
Received on Monday, 18 October 2010 03:45:24 UTC

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