Re: Information Preserving and ISSUE-42

Enrico

I see you point and I agree. Please ignore my previous comment. I was the
one who was confused.

Furthermore, by translating the NULLs, it could into some incorrect
inferences (please see Richard's email in [1]).

So what is your exact proposal for the direct mapping?

[1] http://lists.w3.org/Archives/Public/public-rdb2rdf-wg/2011May/0071.html

Juan Sequeda
+1-575-SEQ-UEDA
www.juansequeda.com


On Wed, May 18, 2011 at 8:46 AM, Enrico Franconi <franconi@inf.unibz.it>wrote:

> On 18 May 2011, at 15:33, Juan Sequeda wrote:
>
> On Wed, May 18, 2011 at 2:30 AM, Enrico Franconi <franconi@inf.unibz.it>wrote:
>
>> On 18 May 2011, at 06:59, Juan Sequeda wrote:
>>
>> > So does that mean that you would translate all NULL values to a triple
>> with rdb2rdf:NULL? That makes sense and would make the current direct
>> mapping information preserving.
>>
>> Definitely not.
>> Take the case of a RDB with a Person table, having ID, name, and age.
>> If you query (in SQL with a simple conjunctive query) all the people with
>> the same age of a specific person in the table having a NULL value as age,
>> you do *not* get the people whose age is a NULL value. However, with your
>> naive translation in RDF of this database, you would get the wrong answer
>> with the same query translated in SPARQL.
>>
>
> Yes, if you have a query like the one you are stating. But for the direct
> mapping, the input is the complete table. So it shouldn't be a problem...
> right?
>
>
> I don't get what you are saying, sorry :-(
> A simple BGP is enough to cover this example.
> In other words, If I do translate the NULL value as a constant, and I try
> to use a BGP to write a query giving me the people with the same age as a
> person with a NULL value as an age, I will fail miserably if I don't
> explicitly mention in the query the special case of the NULL value (which,
> by the way, goes beyond BGPs).
> cheers
> --e.
>
>