SPARQL Blank Nodes
I recently ran across what I believe to be a mistake in the SPARQL 1.1 Query Language, and thought I’d add some detail here.
Section 5.1.1 of the SPARQL 1.1 specification says:
When using blank nodes of the form _:abc, labels for blank nodes are scoped to the basic graph pattern. A label can be used in only a single basic graph pattern in any query.
However, during the parsing of a SPARQL property paths into the SPARQL algebra, many property path expressions do not result in basic graph patterns. Only those expressions that result in “adjacent triple patterns” produce a basic graph pattern. That means that a graph pattern such as:
_:s <p> ?x ;
<q>* ?y .
does not result in a BGP. Intuitively, I think this should be allowed. It’s intention seems clear. However, it results in two primary algebraic components: a basic graph pattern with the triple pattern :s <p> ?x, and a property path :s ZeroOrMorePath(<q>) ?y. This certainly breaks the rule about only using blank nodes in a single basic graph pattern.
The language in section 5.1.1 originated in SPARQL 1.0, and I believe was just overlooked during the update to the language that added property paths.
When handling blank node labels, instead of following the exact language of the specification, I believe SPARQL implementations should instead allow blank node labels that appear in any adjacent set of basic graph patterns and property paths.
Andy Seaborne helpfully added this issue to the SPARQL 1.1 Errata.