Re: Q: ISSUE-41 bNode semantics

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?

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 15:15:02 UTC