- From: Enrico Franconi <franconi@inf.unibz.it>
- Date: Wed, 18 May 2011 15:32:46 +0200
- To: Michael Hausenblas <michael.hausenblas@deri.org>
- Cc: Richard Cyganiak <richard@cyganiak.de>, Ivan Herman <ivan@w3.org>, Pat Hayes <phayes@ihmc.us>, W3C RDB2RDF <public-rdb2rdf-wg@w3.org>
On 18 May 2011, at 15:23, Michael Hausenblas wrote: > We're doing SPARQL 1.0 here in RDB2RDF land, so all the worries re aggregation and updates are moot. Does this change your proposal? Not really, because of (a) & (b) combined. --e. > On 18 May 2011, at 14:20, Enrico Franconi wrote: > >> >> On 18 May 2011, at 13:28, Richard Cyganiak wrote: >> >>> So unless someone (Ted? Enrico?) can propose a better alternative, I'm still in favour of simply not producing triples for NULLs. >> >> 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". >> >> From a meaningful translation of a RDB in RDF, we should be able also to understand the translation of operations (e.g., SQL queries or updates) over the original data in, say, SPARQL over the translated data. I am not interested in the reverse mapping, but of course I'm interested in how to use correctly the data. >> >> If the original RDB data does not contain nulls, and the direct mapping is employed, then it is sort of obvious how to translate the SQL operations into SPARQL operations: basically it goes through reification of the relational signature into an object model. >> However, when NULL values are present, then operations over data (queries, updates) became less obvious. >> >> Examples: >> >> (a) projection over attributes containing NULL values should return the NULL values, different from not returning anything; >> >> (b) a (self-)join fails for tuples with a NULL value in the join attribute; >> >> (c) aggregation, updates, etc. >> >> By not translating NULL values, you fail (a). >> By translating NULL values, you fail (b). >> (c) is even more complex. >> >> How does SQL solve the matter? By considering a NULL value as a constant, and then tweaking the query answering mechanism letting the join fail whenever this constant is found (see the "three valued semantics"). >> >> 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 how SQL normatively defines the NULL values. Note that this may not be a trivial exercise, due to the complexity of the new SPARQL language, which I understand contains aggregations :-( >> >> cheers >> --e. > >
Received on Wednesday, 18 May 2011 13:33:15 UTC