Re: Reverse Mapping RDF2RDF

On Nov 20, 2010, at 16:23 , Alexandre Bertails wrote:

> 
>> I believe we see rdb2rdf as in scope, rdf2rdb as out of scope but if someone out there wants to tool on it and advise us, great. I expect we'll all have our strategies for sparql2sql, and that while it's not up to RDB2RDF to mandate one, it is our job to make sure that our rdb2rdf mapping language enable sparql2sql. 
> 
> We have to ensure that any rdb2rdf solution (direct mapping, datalog
> rules, r2rml) can enable sparql2sql.

... in the vast majority of cases. I would not be shocked as a user if for some weird RDB structures the only way to handle them on the Semantic Web would be to materialize them in RDF via r2rml and take it from there for SPARQL. Note that I am _not_ a database expert, do not ask me to come up with an example!

> 
>>>  I was just wondering whether there was a systematic consideration whether that is possible at all if I write an R2RML or use the direct mapping; if not, under which circumstances, and whether this is something that the author of an R2RML instance can influence. I saw in the inverseExpression term in R2RML; is that enough for what I meant?
>>> 
>>> Maybe some sort of primer text should include more information on that.
>> 
>> There's some commented text in both directMapping and UC&R showing "equivalence" of SPARQL and SQL queries over the leading examples in those documents.
> 
> The relation between RDB, RDF, SPARQL and SQL should be formalized and
> should not rely only on examples. It's called semantics preservation
> -- a well-known problem in the compilation field -- and IMO it should
> be in a normative section.
> 
> Eirc and I are totally confident that in the case of the direct
> mapping, we can map any SPARQL query to its equivalent SQL query.

I would like to meet this Eirc guy, he might be a good addition to the working group:-)

Ivan


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Ivan Herman, W3C Semantic Web Activity Lead
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