[Rdfapi-php-interest] type of non-existant values in optional sparql queries

[Christian W. <cw...@cw...>]

Hello all,

The current memory-SparqlEngine implementation is somewhat wrong when it
comes to OPTIONAL statements that do return no values for some
variables. Let me explain:
Query:
PREFIX  foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?mbox ?name {
    ?x foaf:mbox ?mbox .
    OPTIONAL { ?x foaf:name  ?name } .
}

Data:
@prefix foaf:       <http://xmlns.com/foaf/0.1/> .
_:e foaf:mbox   <mailto:ev...@ex...> .


When querying, variable ?mbox is set but ?name is not, since it isn't in
the database. The correct value of ?name in my eyes is NULL.
SparqlEngine returns an empty string "" which is ambiguous.

I opt for changing the unit tests and adapting the sparql engine. Any
objections?


--=20
Regards/Mit freundlichen Gr=C3=BC=C3=9Fen
Christian Weiske