GROUP BY or GROUP CONCAT does not work

[ocorcho]

When trying to use in a query the GROUP_CONCAT clause, the queries that are produced do not provide any result. As an example, this is a query that we are trying to use for this purpose:

`
PREFIX era: http://data.europa.eu/949/
PREFIX gsp: http://www.opengis.net/ont/geosparql#
PREFIX geo: http://www.w3.org/2003/01/geo/wgs84_pos#
PREFIX rdfs: http://www.w3.org/2000/01/rdf-schema#
PREFIX skos: http://www.w3.org/2004/02/skos/core#
PREFIX dc: http://purl.org/dc/elements/1.1/

SELECT DISTINCT ?sol ?solID ?solName ?inCountry ?length ?ValidityDateStart ?ValidityDateEnd ?solNature ?LineIdentification ?IMCode ?opStartName ?opStartType ?opStartUOPID ?opEndName ?opEndType ?opEndUOPID (GROUP_CONCAT(DISTINCT ?resultTrack; separator = " , ") AS ?trackInfo)
WHERE {
GRAPH http://data.europa.eu/949/graph/rinf {
?sol a era:SectionOfLine . BIND (REPLACE(STR(?sol), "^./([^/])$", "$1") as ?solID) .
?sol rdfs:label ?solName .
?sol era:inCountry ?inCountryURI . BIND (REPLACE(STR(?inCountryURI), "^./([^/])$", "$1") as ?inCountry) .
?sol era:length ?length .
OPTIONAL{?sol era:validityStartDate ?ValidityDateStart .}
OPTIONAL{?sol era:validityEndDate ?ValidityDateEnd .}
?sol era:solNature ?solNatureURI .
?sol era:lineNationalId ?LineIdentificationURI . BIND (REPLACE(STR(?LineIdentificationURI), "^./([^/])$", "$1") as ?LineIdentification) .
?sol era:imCode ?IMCode .
?sol era:opStart ?opStart .
?opStart rdfs:label ?opStartName .
?opStart era:opType ?opStartTypeURI .
?opStart era:uopid ?opStartUOPID .
?sol era:opEnd ?opEnd .
?opEnd rdfs:label ?opEndName .
?opEnd era:opType ?opEndTypeURI .
?opEnd era:uopid ?opEndUOPID .
OPTIONAL {
?sol era:track ?solTracks . BIND (REPLACE(STR(?solTracks), "^./([^/])$", "$1") as ?solTracksURI) .
?solTracks era:trackId ?SOLTrackIdentification .
OPTIONAL{?solTracks era:validityStartDate ?SOLTrackValidityDateStart .}
OPTIONAL{?solTracks era:validityEndDate ?SOLTrackValidityDateEnd .}
OPTIONAL{?solTracks era:imCode ?SOLTrackIMCode .}
OPTIONAL{?solTracks era:trackDirection ?SOLTrackDirectionURI}
BIND (CONCAT("{"SOLTrack":"",?solTracksURI,"","SOLTrackIdentification":"",?SOLTrackIdentification,"","ValidityDateStart":"",?SOLTrackValidityDateStart,"","ValidityDateEnd":"",?SOLTrackValidityDateEnd,"","SOLTrackDirection":"",?SOLTrackDirection,"","SectionOfLineID":"",?solID,""}") AS ?resultTrack) . }}
GRAPH http://data.europa.eu/949/graph/skos {
?opStartTypeURI skos:prefLabel ?opStartType . FILTER (lang(?opStartType) = 'en') .
?opEndTypeURI skos:prefLabel ?opEndType . FILTER (lang(?opEndType) = 'en') .
?solNatureURI skos:prefLabel ?solNature .
?SOLTrackDirectionURI skos:prefLabel ?SOLTrackDirection}
} GROUP BY ?sol ?solID ?solName ?inCountry ?length ?ValidityDateStart ?ValidityDateEnd ?solNature ?LineIdentification ?IMCode ?opStartName ?opStartType ?opStartUOPID ?opEndName ?opEndType ?opEndUOPID
`