- From: Richard Cyganiak <richard@cyganiak.de>
- Date: Tue, 24 May 2011 12:29:18 +0100
- To: Marcelo Arenas <marcelo.arenas1@gmail.com>
- Cc: W3C RDB2RDF <public-rdb2rdf-wg@w3.org>
On 24 May 2011, at 12:00, Richard Cyganiak wrote: > Unfortunately, we know that there cannot be a semantics preserving mapping from relational databases to RDF in the presence of NULLs. NULLs in SQL can indicate the absence of a value, and negation cannot be expressed in RDF. Thinking more about it, this might not be true. NULL in SQL means: “Either this has an unknown value, or it has no value.” I'll try to restate that in logical terms. A(?x, NULL) means: There exists some ?y such that A(?x,?y) or there exists no ?y such that A(?x,?y). That says exactly nothing. It is trivially true. So, not creating a triple for NULL is actually semantics-preserving. Or is it? This sounds too simple to be true, so I probably made some stupid mistake. Best, Richard
Received on Tuesday, 24 May 2011 11:29:46 UTC