- From: Bijan Parsia <bparsia@cs.man.ac.uk>
- Date: Mon, 20 Dec 2010 17:30:36 +0000
- To: Enrico Franconi <franconi@inf.unibz.it>
- Cc: SPARQL Working Group <public-rdf-dawg@w3.org>, Maurizio Lenzerini <lenzerini@dis.uniroma1.it>
Hi Maurizio,
Thanks for the message!
On 20 Dec 2010, at 09:48, Enrico Franconi wrote:
> I forward this message I received from Maurizio Lenzerini.
>
> Begin forwarded message:
[snip]
>> In all the applications mentioned above, there is a strong need of answering queries with non-distinguished variables. Just to name one interesting scenario where missing non-distinguished variables would be a real problem, consider checking quality/completeness of data.
>>
>> The query:
>>
>> { x,z | R1(x,y), R2(y,z) }
>>
>> tells me which x and z are connected through y, without necessarily knowing who is the y. On the other hand, the query
>>
>> { x,y,z | R1(x,y), R2(y,z) }
>>
>> tells me for which x,z I KNOW the y.
As an example, this isn't really very informative. It's a fake toy example which merely illustrates the difference between the two. We're all well aware of the semantic differences --- we're trying to find real cases we can study.
And I seem to be missing something: Shouldn't a *data* completeness scenario be covered by the second query (perhaps with projection)? That is, for the data to be *complete*, don't I need it to be the case that x and z are connected via a bindable name? If the connection is merely existential then the data is incomplete, right? Or am I missing something?
If I just don't want to see the y, then leaving y ranging over the active domain is fine: Projection drops that column.
Thanks for your help!
Cheers,
Bijan.
Received on Monday, 20 December 2010 17:31:11 UTC