PREFIX schema: <http://qweb.cs.aau.dk/airbase/schema/>
PREFIX property: <http://qweb.cs.aau.dk/airbase/property/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX qb: <http://purl.org/linked-data/cube#>
SELECT ?type ?city ?pollutant (avg(?no2) as ?avgno2)  WHERE {
    ?s property:type ?type  .
    ?obs schema:station ?s .
    ?obs rdf:type qb:Observation .    
    ?obs ?p ?no2 .
    ?s schema:inCity ?city .
    ?obs schema:sensor ?sen .
    ?sen schema:measures ?comp .
    ?comp property:caption ?pollutant .
	FILTER (isLiteral(?no2))
} GROUP BY ?type ?city ?pollutant