- From: Arjohn Kampman <arjohn.kampman@aduna.biz>
- Date: Fri, 14 Nov 2003 16:35:37 +0100
- To: Damian Steer <pldms@mac.com>
- Cc: Jeen Broekstra <jeen@aduna.biz>, "Seaborne, Andy" <Andy_Seaborne@hplb.hpl.hp.com>, 'Steve Harris' <S.W.Harris@ecs.soton.ac.uk>, www-rdf-interest@w3.org, sesame-devel@lists.sourceforge.net
Damian Steer wrote: [...] >>>Hi there - I have an example query with optional triples and I wondered what >>>the various systems do with it: >>>Thanks to Jeremy Carroll for this example. >>>Consider the data: >>><x> <p> <y> . >>><x> <q> <z> . >>>and the query: >>>[ <x> <p> ?a ] >>>[ <x> <q> ?a ] >>>where [] is an optional match. >>>?? Does the query match the data? [...] >>>?? What does it return? >> >>I'm heavily leaning towards saying that the query engine should return >>a "malformed query" error. >> >>In the discussion on sesame-devel, Jacco van Ossenbruggen came up with >>a constraint on optional path expressions: reuse of a variable across >>optional paths is allowed if and only if the variable is also used in >>a non-optional path in the same query. This disambiguates the use of >>the shared variable in optionals, since optional paths will no longer >>have to instantiate the shared variable, they will only have to >>validate the current instantiation. > > > Yes. That was my conclusion to the issue - essentially 'sanity > checking'. It doesn't seem unreasonable. Unless someone is able to give a plain-English description of what this query is meant to return, i.e. a sentence starting with something like "Give me all values that...". We were not able to come up with such a description. Anyone? > I should add that I did consider returning graphs rather than variable > bindings, which has the virtue that the results are > unambiguous. However this, in my case, was essentially useless as I > was trying to retrieve information from a graph, and returning a graph > seemed a little perverse. Amen. This is exactly why SeRQL offers both: you need table-like query results in the end because otherwise you stuck with graphs forever. Cheers, Arjohn -- arjohn.kampman@aduna.biz Aduna B.V. (formerly known as aidministrator) - http://www.aduna.biz/ prinses julianaplein 14-b, 3817 cs amersfoort, the netherlands tel. +31-(0)33-4659987 fax. +31-(0)33-4659987
Received on Friday, 14 November 2003 11:39:06 UTC