PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> PREFIX sioc: <http://rdfs.org/sioc/ns#> PREFIX sioct: <http://rdfs.org/sioc/types#> PREFIX skos: <http://www.w3.org/2004/02/skos/core#> SELECT DISTINCT ?tag ?topic WHERE { ?forum a sioct:Wiki . ?forum sioc:container_of ?post. OPTIONAL { ?post sioc:topic ?topic }. OPTIONAL { ?topic rdf:type skos:Concept }. OPTIONAL { ?topic skos:prefLabel ?tag } }