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ISSUE-20: SPARQL Update: Empty vs non-existent graphs

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Wed, 23 Dec 2009 11:33:15 +0000
Message-ID: <4B31FFFB.8020009@talis.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
ISSUE-20

Pulled out because this interacts with query (1.0).

 > ISSUE-20: Difference between an empty graph and a non-existent graph?
 > We have agreed on the need to support a graph that exists and is
 > empty. Is there anything else to consider for this issue? Does it have
 > an impact on the Update document? By coming to this agreement, then I
 > think we*don't*  need to worry about it in Update. We'd only have to
 > consider it if we disallowed empty graphs.
 >
 > I've left ISSUE-20 in, but if people agree that it's resolved, then
 > I'll take it out.

My preference is that empty graphs and absent graphs are treated 
differently.  This is based on seeing users expectations of systems I'm 
involved in.

It does make store writing harder and some of mine don't support this 
because they can only test for the presence or absence of quads.  There 
is no separate graph management.  No quads does not mean no graph, if 
empty graphs can exist.

It can show up in query in
    SELECT * { GRAPH ?g {...} }

Some related issues in update are about adding triples into an empty 
graph vs a non-existent graph (does it automatically get created?) 
whether CREATE on an existing (empty of otherwise) graph is an error and 
DROPping a graph not in the store.

	Andy
Received on Wednesday, 23 December 2009 11:33:40 GMT

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