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Re: Comments on SPARQL Query Language for RDF (21 July 2005 version)

From: Dan Connolly <connolly@w3.org>
Date: Thu, 01 Sep 2005 13:01:57 -0500
To: Art.Barstow@nokia.com
Cc: public-rdf-dawg-comments@w3.org, Ora.Lassila@nokia.com
Message-Id: <1125597717.16011.699.camel@dirk>

On Thu, 2005-09-01 at 13:12 -0400, Art.Barstow@nokia.com wrote:
> Regarding the 2005-07-21 version of the SPARQL Query 
> Language for RDF document, we have the following comments:
> 
> 1) This document does not discuss in any way the 
> *semantics* of the query language. We would like to see 
> a more formal definition of queries (and their results) 
> in terms of RDF semantics (right now, the query language
> seems to treat RDF graphs as merely data structures from 
> which something can be extracted). Why would SPARQL now 
> ignore RDF's model theory when one was created through 
> a sizeable effort?

Could you be more specific about what's lacking? What
questions should be answered that are not currently
answered?

I suppose we could make explicit that for a query pattern P,
if S is a solution w.r.t. an input graph G,
then S(P) is entailed by G. Is that what you have in mind?

I think the idea can be expanded to cover UNION straightforwardly,
and perhaps OPTIONAL with some effort, but I don't know how this
applies to queries that use the GRAPH keyword.

RDF's model theory was not ignored. There is a normative dependency
from SPARQL to the RDF model theory in section 7.1.

The link was not as clear as it could be in the 2005-07-21 version;
In response to comments from other reviewers, we have clarified
the references section. In the current editor's draft, you
may want to look at...
  http://www.w3.org/2001/sw/DataAccess/rq23/#RDF-MT


> 2) Given that RDF representations -- effectively -- are 
> graphs, why would the W3C present a query language based 
> on relational algebra? It is well known [1] that relational 
> algebra is insufficient for querying graphs (generally, 
> data structures that exhibit repetitive or recursive patterns). 
> In order to query, say, hierarchies of arbitrary depth, the 
> query language should have some means of expressing 
> a transitive closure.

The WG began its design discussion by surveying
known technologies:
 http://www.w3.org/2001/sw/DataAccess/DesignEvaluations

Versa was among the designs we considered, and it's fairly strongly
path-based. It got some support, but not a critical mass.

The level of support for various designs was discussed at
the 2nd ftf meeting a few times before we eventually
chose BRQL, which is largely relational.
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn2
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn3

In the use cases we have explored, where transitive closure
is needed, the query is run over a notional background
graph that includes the inferred transitive closure.
By charter, this sort of inference is orthogonal to
query. More on that below.


> 
> 3) It does not seem possible to extend SPARQL to be 
> used with OWL (primarily, perhaps, because of comment
> #1 above).

A number of WG members (UMD, Agfa) are succesfully using SPARQL
with OWL.

By charter, OWL inference is orthogonal to query:

[[
2.1 Specification of RDF Schema/OWL semantics

The protocol will allow access to a notional RDF graph. This may in
practice be the virtual graph which would follow from some form of
inference from a stored graph. This does not affect the data access
protocol, but may affect the description of the data access service. For
example, if OWL DL semantics are supported by a service, that may be
evident in the description of the service or the virtual graph which is
queried, but it will not affect the protocol designed under this
charter.
]]
http://www.w3.org/2003/12/swa/dawg-charter#rdfs-owl-queries


> Regards,
> 
> Art Barstow
> ---
> 
> [1] Aho, A.V., Ullman, J.D.: Universality of data retrieval 
> languages. In: POPL 79: Proceedings of the 6th ACM SIGACT-SIGPLAN 
> symposium on Principles of programming languages, ACM Press (1979) 
> 110119.
-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
Received on Thursday, 1 September 2005 18:02:02 GMT

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