Re: Q: ISSUE-41 bNode semantics

On Wed, 2011-05-18 at 17:24 +0200, Enrico Franconi wrote:
> On 18 May 2011, at 17:14, Alexandre Bertails <bertails@w3.org> wrote:
> 
> > On Wed, 2011-05-18 at 17:05 +0200, Enrico Franconi wrote:
> >> If this is the attitude of the majority of the group, I will then request that the mapping explicitly does not deal with RDBs having NULL values, and that will produce results incompatible with the RDB semantics for NULL values otherwise. If you do not say this explicitly, then this group will produce a standard that will not be backward compatible whenever in the future you will have to deal with NULL values.
> >> My opinion is that we have to consider NULL values, since I barely know real world RDBs without NULL values.
> > 
> > This is still a claim. Can you please provide a use-case, some arguments
> > and give us some proposals?
> 
> I believed I already did in my emails, but I can resummarise my claims and my proposal later today when I will be back home. As for the claim that most *real world* (I mean in companies with non-toy RDBs and real DBAs, etc) RDBs contain NULLs I'll leave it as it is.

I believe that the NULL thing was brought by a discussion about the
Information Preservation concern. We have already said this will be
handled by the mapping of the RDB schema.

More specifically about handling NULL values... We *did* take that into
account by formally deciding not to map a triple for a value that does
not exist.

So my questions are:
* what is your requirement for NULL values?
* what do you expect to see in the RDF?
* what's missing today in the Direct Mapping?
* do you have concrete examples?
* can you be specific about which part(s) of the Direct Mapping must be
fixed?

Alexandre.

> 
> cheers
> --e.
> 
> > 
> > Alexandre.
> > 
> >> Cheers
> >> --e.
> >> 
> >> I don't understand on which grounds you claim that this can not be done - at least in a simplified context.
> >> I strongly disagree on your statement on CWA and 3-valued logic; indeed relational algebra does deal with SQL NULLs by introducing the is-not-null predicate.
> >> 
> >> On 18 May 2011, at 16:54, Richard Cyganiak <richard@cyganiak.de> wrote:
> >> 
> >>> On 18 May 2011, at 14:20, Enrico Franconi wrote:
> >>>> Please let me note first that my arguments are not about "what a NULL value possibly does mean among various possibilities", but they are about "what a NULL value normatively means in the SQL standard".
> >>> 
> >>> RDF cannot express the complete semantics of SQL NULLs. A complete “direct mapping” is not possible without changing the semantics of RDF to closed-world and adding three-value logic to SPARQL. I believe that this WG has not been chartered to do that.
> >>> 
> >>>> To mimic this in RDF2RDF, my suggestion would be to translate a NULL value as a special constant from a special datatype, and then we should provide precise directives on how a query language should deal with this.
> >>> 
> >>> This is not a good solution. As far as I know, no one implements anything like it, and no implementer or user is requesting it. I conclude that it is not needed and a waste of WG resources.
> >>> 
> >>> Best,
> >>> Richard
> >> 
> >> 
> > 
> > 
> > 
>

Received on Wednesday, 18 May 2011 15:41:31 UTC