Hi all,
I recently posted an answer to a Stack Overflow question [1] that led
me to write the query
select ?prop ?c
where {
dbpedia:Milano dbpedia-owl:wikiPageRedirects* [ ?prop ?c ] .
}
and submit it to the DBpedia public endpoint [2] (which has the
appropriate namespace prefixes defined), but I get this error message:
"""
Virtuoso 37000 Error SP031: SPARQL compiler: Object of transitive
triple pattern should be variable or QName or literal, not blank node
SPARQL query:
define sql:big-data-const 0
#output-format:text/html
define sql:signal-void-variables 1 define input:default-graph-uri
<http://dbpedia.org> select ?prop ?c
where {
dbpedia:Milano dbpedia-owl:wikiPageRedirects* [ ?prop ?c ] .
}
"""
If I explicitly define the prefixes
prefix dbpedia: <http://dbpedia.org/resource/>
prefix dbpedia-owl: <http://dbpedia.org/ontology/>
select ?prop ?c
where {
rdf:Milano rdf:wikiPageRedirects* [ ?prop ?c ] .
}
and bring it over to sparql.org's validator, I get no complaints, and
it says the query is equivalent to
1 PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
2 PREFIX dbpedia: <http://dbpedia.org/resource/>
3
4 SELECT ?prop ?c
5 WHERE
6 { dbpedia:Milano (dbpedia-owl:wikiPageRedirects)* _:b0 .
7 _:b0 ?prop ?c
8 }
which makes sense, based on 4.1.4 Syntax for Blank Nodes [4] from the
spec which says:
"""
The following two forms
[ :p "v" ] .
[] :p "v" .
allocate a unique blank node label (here "b57") and are equivalent to writing:
_:b57 :p "v" .
"""
I asked about it on the Jena-users list (since the validator is
Jena-based), and got this response from Andy Seaborne:
"""
It's legal syntax.
[[
Virtuoso 37000 Error SP031: SPARQL compiler: Object of transitive
triple pattern should be variable or QName or literal, not blank node
]]
suggests to me it is not an error in parsing but in
translation/compilation afterwards; it's quite specific to the form
you've used. It might be doing something special for "property*".
> ObjectListPath
> ObjectPath
ObjectPath ::= VarOrTerm | TriplesNodePath
VarOrTerm covers the [] case, not the [ ... ] case.
TriplesNodePath ::= CollectionPath | BlankNodePropertyListPath
BlankNodePropertyListPath ::= '[' PropertyListPathNotEmpty ']'
PropertyListPathNotEmpty ::= ... VerbSimple ObjectListPath ...
matches "?p ?c"
so [ ?prop ?c ] is OK.
"""
Is this a bug in Virtuoso, or something that's a product of some
configuration, or DBpedia using an older version, or… ?
Thanks,
//JT
[1] http://stackoverflow.com/q/19380175/1281433
[2] http://dbpedia.org/sparql
[3] http://www.w3.org/TR/sparql11-query/#rGraphTerm
[4] http://www.w3.org/TR/sparql11-query/#QSynBlankNodes
[5]
http://mail-archives.apache.org/mod_mbox/jena-users/201202.mbox/%3c4f4019b6.1030...@apache.org%3E
--
Joshua Taylor, http://www.cs.rpi.edu/~tayloj/
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