Hi David, the empty mapping is compatible with any other mapping, thus also with Ω(?var->i).
best, Axel -- Dr. Axel Polleres url: http://www.polleres.net/ twitter: AxelPolleres On 14 Aug 2013, at 15:34, David Booth <[email protected]> wrote: > [Copying Andy Seaborne and Steve Harris for their input.] > > Hi Axel, > > The part of the spec that I was trying to bring to your attention was where > it says that the empty group graph pattern "does not bind *any* variables" > (my emphasis). If ?G is not bound then that short form of listing existing > graphs would not work. But I am not sure from reading the spec whether ?G is > supposed to be bound or not. The SPARQL algebra says: > > http://www.w3.org/TR/sparql11-query/#defn_evalGraph > [[ > eval(D(G), Graph(var,P)) = > Let R be the empty multiset > foreach IRI i in D > R := Union(R, Join( eval(D(D[i]), P) , Ω(?var->i) ) > the result is R > ]] > Hmm, there's right parenthesis missing, which I guess I'll report separately. > > It looks like Ω(?var->i) is binding the graph variable, but then it's doing a > "Join", which is defined as: > http://www.w3.org/TR/sparql11-query/#defn_algJoin > [[ > Join(Ω1, Ω2) = { merge(μ1, μ2) | μ1 in Ω1and μ2 in Ω2, and μ1 and μ2 are > compatible } > ]] > > I couldn't readily find the definition of the merge function, but I think I > found the definition of "compatible": > http://www.w3.org/TR/sparql11-query/#defn_algCompatibleMapping > [[ > Two solution mappings μ1 and μ2 are compatible if, for every variable v in > dom(μ1) and in dom(μ2), μ1(v) = μ2(v). > ]] > which seems to be saying that the variables being joined must be bound to the > same values. But since the empty basic graph pattern does not bind ?G, I > *think* this means that Ω(?var->i) would *not* be compatible and ?G would > therefore not be bound in the result. > > So, if I have properly followed the SPARQL algebra, I *think* this means that > the short form that you suggested for listing graphs will not work, and it is > not possible to get a list of graphs that includes empty graphs. (Hence the > Sesame 2.7.1 behavior is correct.) > > Andy or Steve, have I got this right? > > Thanks, > David > > On 08/13/2013 03:10 PM, Axel Polleres wrote: >>> [[ The group pattern: { } matches any graph (including the empty >>> graph) with one solution that does not bind any variables. ]] >> >> This only means that upon >> >> SELECT ?G WHERE { GRAPH ?G {} } >> >> also empty named graphs should be returned, which would not be the >> case for >> >> SELECT ?G WHERE { GRAPH ?G { ?S ?P ?O } } >> >> Obviously, this makes a difference for all graph stores that support >> empty named graphs. So, to my understanding at least, this is not a >> bug in the spec. >> >> HTH, Axel >> >> On 13 Aug 2013, at 20:21, David Booth <[email protected]> wrote: >> >>> Hi Axel, >>> >>> That doesn't work in Sesame 2.7.1 at least, apparently because ?G >>> is not bound, even though there is one solution. The SPARQL 1.1 >>> spec says: http://www.w3.org/TR/sparql11-query/#emptyGroupPattern >>> [[ The group pattern: { } matches any graph (including the empty >>> graph) with one solution that does not bind any variables. ]] >>> >>> Is this a bug in the spec? >>> >>> David >>> >>> On 08/13/2013 11:48 AM, Axel Polleres wrote: >>>> Hi Quentin, >>>> >>>> how about just >>>> >>>> SELECT ?G WHERE { GRAPH ?G {} } >>>> >>>> (no need to dump all triples, if the only concern is which ?G >>>> exist) >>>> >>>> BTW, [email protected] may be the list you wanted to use. >>>> >>>> best, Axel >>>> >> >> >> >>
