RDFLib/rdflib (original) (raw)

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GROUP_CONCAT with OPTIONAL returns 'None' #1467

Description

A query such as the following on https://hitontology.eu/sparql returns empty strings for nonexisting GROUP_CONCAT results using Virtuoso:

PREFIX dce: <http://purl.org/dc/elements/1.1/>
PREFIX hito: <http://hitontology.eu/ontology/>
SELECT
?classified
(GROUP_CONCAT(DISTINCT(STR(?dce_source));separator="|") AS ?dce_sources)
{
 ?classified a hito:FeatureClassified.
 OPTIONAL {?classified dce:source ?dce_source.}
}

However when querying an RDFLib graph, I get rdflib.term.Literal('None') values in my result set. Is that intended?