Re: Non-PK tables, blank nodes, and leanness

Hi Ashok,

On 24 Apr 2012, at 22:07, ashok malhotra wrote:
> It seems clear that DM should generate 
> 
>   _:1 <IOU#BORROWER> "Alice".
>   _:1 <IOU#AMOUNT> 10.
>   _:2 <IOU#BORROWER> "Alice".
>   _:2 <IOU#AMOUNT> 10.
> 
> If that's what the spec says, we are done.

This assertion is based on what?

To the best of my knowledge, the version above can only be implemented by either materializing the table, or by using Oracle-specific extensions, and it is incompatible with R2RML. (Someone smarter than me might still prove me wrong on this.)

There is another semantically equivalent version that is much easier to implement with just vanilla SQL, and works with R2RML.

What advantage do you get by choosing the more complex, SQL-incompatible, R2RML-incompatible version?

As far as I can tell, the advantage is that you get a more “natural” RDF graph structure for an infrequent corner case (tables without PK).

That advantage is not worth the cost and loss of compatibility.

I'd be happy with allowing both versions, but given the facts above, I don't think I can agree to forbidding the simpler, SQL-standard-compatible, R2RML-compatible version.

Best,
Richard



> 
> There may well be a RDF semantics/SPARQL issue lurking here but that's another matter.
> All the best, Ashok
> 
> On 4/24/2012 1:13 PM, Juan Sequeda wrote:
>> This is a non lean RDF graph and per the RDF semantics, they are equivalent. 
>> 
>> Gotta love the RDF semantics. 
>> 
>> So, even though they are equivalent per RDF semantics, we still maintain the cardinality. But if we query in SPARQL, we get two different things. Therefore, there is a mismatch between the semantics of SPARQL and RDF. Interesting, eh?
>> 
>> Juan Sequeda
>> www.juansequeda.com
>> 
>> On Apr 24, 2012, at 9:53 PM, David McNeil <dmcneil@revelytix.com> wrote:
>> 
>>> On Tue, Apr 24, 2012 at 1:44 PM, Richard Cyganiak <richard@cyganiak.de> wrote:
>>>   _:1 <IOU#BORROWER> "Alice".
>>>   _:1 <IOU#AMOUNT> 10.
>>>   _:2 <IOU#BORROWER> "Alice".
>>>   _:2 <IOU#AMOUNT> 10.
>>> 
>>> Maybe I don't understand blank nodes properly. I thought the graph above was asserting the existence of two unique resources (since there are two blank node IDs).
>>> 
>>> Thanks.
>>> -David

Received on Tuesday, 24 April 2012 22:33:11 UTC