The evaluation of GRAPH is defined by
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
so it's a loop over
Join(
eval on graph_i of pattern {}
Ω(?var->i)
)
eval on graph_i of pattern {} is a binding od one row, no variables; the
join identity. Joined to a binding of ?var/i and you get ?var/i.
Union it all and it's the set of graph IRIs (no duplicates because the
join identity does not introduce any).
Andy
On 13/08/13 20:10, 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
