Re: Reverse Mapping RDF2RDF

Yes indeed, the vocabulary should be consistent:-)

My question on the CG was on the sparql2sql aspect. I think that, though it might be important in practice, rdf2rdb is out of scope. But having at least some clear directions on sparql2sql would be good although I realize that the efficient implementation of something like that might be a question of market force and competition...

On Nov 20, 2010, at 13:08 , Eric Prud'hommeaux wrote:

> * Ivan Herman <ivan@w3.org> [2010-11-20 11:24+0100]
>> I wonder where this agenda item comes from... I guess it was triggered by my question to Michael at the SWCG call.
>> 
>> I did not have anything very complicated in mind, just a question... we are talking about the possibility of translating SPARQL queries to the SQL calls on-the-fly.
> 
> Ahh, to me "reverse mapping" denotes a mapping from RDF graphs to RDB tables (which has its own use cases and advocates but is outside this charter). As to exploiting an rdb2rdf mapping as a virtual view, I believe that it is what every one of us has done and aims to do interoperably. There were a series of presentations at the beginning of the WG's life 
>  http://www.w3.org/2001/sw/rdb2rdf/wiki/Initial_Round_of_Presentations
> which showcased mostly the different strategies for sparql2sql.

Good. If this has been covered that is fine. Maybe some elements of these should be part of a more public document

> No one as yet has demonstrated sql2sparql.

Which might be an interesting question but probably out of scope...

> 
> For consistency, we could adopt some vocabulary:
>  rdb2rdf: mapping relational data to rdf graphs.
>  rdf2rdb: mapping rdf graphs to relational data.
>  sparql2sql: mapping sparql queries to sql queries.
>  etc.
> 
> I believe we see rdb2rdf as in scope,

obviously:-)

> rdf2rdb as out of scope but if someone out there wants to tool on it and advise us, great.

... and if there is something and we have time an energy, I could go as far as saying that even some sort of a WG note would be in order. Certainly not more than that.

> 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. 

Or at least enables in the 80% of the cases... And some sort of a documentation of that would be in order

Ivan


> 
> 
>>  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. I scare-quote equivalence because SPARQL returns RDF terms and SQL, unless you significantly change the wire protocol, returns strings which can be parsed to RDF terms. 




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Ivan Herman, W3C Semantic Web Activity Lead
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