Re: Information Preserving and ISSUE-42

+1

NULL produce no Triple + Relational schema mapped into RDFS/OWL (Triples
Schema)

Best Regards,

Percy

2011/5/18 Juan Sequeda <juanfederico@gmail.com>

> 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.
>>
>>
>