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Re: non-empty default graphs

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Fri, 03 Jun 2005 17:36:39 +0100
Message-ID: <42A08717.1000504@hp.com>
To: "Eric Prud'hommeaux" <eric@w3.org>
CC: Dan Connolly <connolly@w3.org>, RDF Data Access Working Group <public-rdf-dawg@w3.org>

I noted in a reply to Yoshio

http://lists.w3.org/Archives/Public/public-rdf-dawg/2005AprJun/0296.html

that rq23 was unclear as to make might happen if the service has a dataset AND 
the query specifies one.

I suggested that, in the same way that the protocol overrides the query, the 
query should override the service dataset (or be rejected).

This was the intent behind saying a "dataset description".  If the service uses 
a different dataset to that asked for, it is wrong in some sense.

	Andy

Eric Prud'hommeaux wrote:
> On Thu, Jun 02, 2005 at 09:29:09AM -0500, Dan Connolly wrote:
> 
>>On Thu, 2005-06-02 at 07:35 -0400, Eric Prud'hommeaux wrote:
>>
>>>This is kind of like a test case for fromUnionQuery, hence the
>>>threading choice.
>>>
>>>Using Named Graphs (rq23) parlance, if I have a dataset:
>>>
>>>  default: { }
>>>  <lies.rdf>: { <TheMoon> <isMadeOf> <GreenCheese> }
>>>  <truth.rdf>: { <TheMoon> <isMadeOf> <Rocks>, <Dirt> }
> 
> 
> Perhaps i over-abbreviated. This particular query service has no
> innate data (unlike, say, a database of the CIA World Fact Book).
> There are also two rdf files, lies and truth, out there on the web.
> 
> 
>>>and a query:
>>>
>>>  SELECT ?stuff
>>>  FROM <truth.rdf>
> 
> 
> per rq23, reads <truth.rdf> into the default graph.
> 
> 
>>>  FROM NAMED <lies.rdf>
> 
> 
> reads <lies.rdf> into a named graph called <lies.rdf>.
> 
> 
>>>  WHERE { <TheMoon> <isMadeOf> ?stuff }
>>>
>>>I get pretty predictable results:
>>>
>>>  | Rocks |
>>>  | Dirt  |
>>
>>Huh? Above you said the default graph was empty. So
>>you should get no results.
>>
>>Your example has two conflicting descriptions of
>>the dataset. Pick one, please.
>>
>>If you pick the one suggested by the FROM/FROM NAMED
>>parts of the query, you get:
>>
>>  default: { <TheMoon> <isMadeOf> <Rocks>, <Dirt> }
>>  <lies.rdf>: { <TheMoon> <isMadeOf> <GreenCheese> }
>>  <truth.rdf>: { <TheMoon> <isMadeOf> <Rocks>, <Dirt> }
>>
>>
>>
>>
>>>But the default graph need not start out empty.
>>
>>"start out"? datasets don't change. They just are.
> 
> 
> The notional SPARQL interface to W3C mail search is a service that
> starts out with some amount of innate data. It could be in named
> graphs or in the default graph. Both FROM and FROM NAMED change the
> data that is available to the query.
> 
> 
>>> If i happen on a default
>>>graph that alreay has <lies.rdf> in it:
>>>
>>>  default: { <TheMoon> <isMadeOf> <GreenCheese> }
>>>
>>>and do the same query, I get:
>>>
>>>  | Rocks       |
>>>  | Dirt        |
>>>  | GreenCheese |
>>>
>>>Thus, if the defualt database *may* be contiminated, you need to fall
>>>back to the more verbose form of provenance constraint:
>>>  SELECT ?stuff
>>>  FROM <truth.rdf>
>>>  FROM NAMED <lies.rdf>
>>>  WHERE { GRAPH <truth.rdf> { <TheMoon> <isMadeOf> ?stuff } }
>>>
> 
> 
Received on Friday, 3 June 2005 16:37:08 GMT

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