How many {established from medical condition} are to/by {Down syndrome} ?
Down Syndrome is an established medical condition of what number of children?
How many kids have the established medical condition Down syndrome?
SELECT ( COUNT ( ?d ) AS ?u ) { ?d property:established_from_medical_condition resource:Down_syndrome }
SELECT (COUNT(?sub) AS ?subs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?sub .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P5166> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q47715 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P5166 wd:Q47715 }
144