Hi Maxence,
Blank nodes are anonymous -- the nodes without labels in a visual graph
of your model.
In an RDF store these nodes are given identifiers in order to include
the nodes in statements. The actual value of a blank node's identifier
though is local to the graph it is part of. Generating another graph via
a query is very likely to assign different values, and even two
identical queries against the same graph are likely to return different
blank node identifiers.
Regards,
Dave
On 21/02/12 9:27 PM, Maxence Guesdon wrote:
Hello,
I have a problem when using the result of a CONSTRUCT query. I do not know
if the behaviour I get is the one expected according to the sparql
definition (I did not find this case in [1]).
I execute the following query:
(I know that<http://foo> is not a correct uri, it is just to make my
example shorter in this message)
CONSTRUCT {
<http://subject> <http://predicate> ?seq .
?seq ?seq_index ?uri .
?uri<http://resource> ?uri2
}
WHERE
{<http://subject> <http://predicate> ?seq .
?seq ?seq_index ?uri
OPTIONAL { ?uri<http://resource> ?uri2 }
}
In my model,
?seq will match a blank node and
?seq_index will match rdf:_N nodes.
?uri can match an uri or a blank node, for I have in my model
...<rdf:_N> <uri>
but sometimes
...<rdf:_N> _:foo
_:foo<http://listOf> <uri>
When I execute this query on my model, in the resulting triples, ?seq is
bound to a *new* blank node, not the original blank node of my model. I
thought blank nodes would be introduced in the result triples only if I
used blank node syntax (like [], _:foo and so on). But it appears that
fresh blank nodes are also introduced when a variable is bound to a blank
node of the model.
For example, I was expecting to have the triple
<http://subject> <http://predicate> _:r1329810294r9040r2
but instead I get
<http://subject> <http://predicate> _:r1_r1329810294r9040r2
Is this the expected behaviour ?
Regards,
Maxence
[1] http://www.w3.org/TR/rdf-sparql-query/
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