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Re: issues in rq25

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Wed, 24 Jan 2007 17:35:18 +0000
Message-ID: <45B798D6.8020303@hp.com>
To: Eric Prud'hommeaux <eric@w3.org>
CC: public-rdf-dawg@w3.org



Eric Prud'hommeaux wrote:
> * Eric Prud'hommeaux <eric@w3.org> [2007-01-23 13:44+0100]
>> I am happy to be working with Andy's new draft, rq25. I've done an
>> editing pass through the end of chapter 7. I have put my
>> characteristic pink issue hilighter on a few points which I am
>> describing here.
>>
>>
>> 4.1.4 Syntax for Blank Nodes
>>
>>   Blank nodes in query patterns act as *existential* variables.
>>
>> I thought they acted as regular variables, i.e. one gets solutions for
>> each way the bnode could match a different term in the graph.
>> [TST] (below) tests this.
>>
>>   The scope of the label is the basic graph pattern; if the same label
>>   is used in another basic graph pattern graph pattern, it is not the
>>   same blank node.
>>
>> Is there really (still? again?) no correlation in the _:who variables
>> in this query?
>>
>> Data:
>>   [ foaf:nick "ericP";
>>     foaf:mboxMD5 "A2BA23432B434443D45DF655A6C6E6E" ].
>>   [ foaf:name "Bob Smith";
>>     foaf:mbox <mailto:bob@example.com> ].
>>
>> Query:
>>   SELECT ?nick ?mbox
>>    WHERE { _:who foaf:mboxMD5 "A2BA23432B434443D45DF655A6C6E6E";
>> 		   foaf:nick ?nick
>> 	   OPTIONAL { _:who foaf:mbox ?mbox } }
>>
>> Results:
>>   ?name  ?mbox
>>   ericP  <mailto:bob@example.com>
> 
> Andy proposed in the meeting to make this misleading query (in that it
> associates ericP's name with Bob Smith's mbox) illegal by prohibiting
> the re-use of BNodes between different BGPs. The cost is to the person
> who does a SELECT * and then limits their returned fields by
> judiciously using BNodes for the fields they don't want to see. Not a
> big cost.

Minor correction: no reuse of *bNode labels*

At the moment, bNode labels are scoped to the BGP so those 2 bnode labels 
_:who, are the same label, but, becaus ethey are in different BGPs, they 
become different bNodes as things stand.

That's the confusion that may arise that disallowing reuse catches.

	Andy
Received on Wednesday, 24 January 2007 17:35:29 GMT

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