SPARQL/Federated query

SPARQL
Federated query

Federated query

Federated query is the ability to take a query and provide solutions based on information from many different sources.

A building block is the ability to have one query be able to issue a query on another SPARQL endpoint during query execution.

Example

Federated query comparing Wikidata and UK Parliament database, to show differences of more than 10km in locations of UK parliamentary constituencies (source Wikidata:Wikidata:Status updates/2019 06 10)

# compare lat/long of Parliament and Wikidata constituency records
#defaultView:Map{"hide":["?line"]}
PREFIX parliament:<https://id.parliament.uk/schema/>

SELECT DISTINCT ?constituency ?parlcoord ?item ?itemLabel ?wdcoord ?dist ?line WHERE {
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
  SERVICE <https://api.parliament.uk/sparql> 
    { ?constituency parliament:constituencyGroupHasConstituencyArea ?area .
      ?area parliament:latitude ?lat . ?area parliament:longitude ?long . 
      bind(SUBSTR(str(?constituency),26) as ?parlid) . }
  BIND(concat("Point(",str(?long)," ",str(?lat),")") as ?parlcoord) 
  # get constituencies from Parliament with coordinates
  ?item wdt:P6213 ?parlid . ?item wdt:P31 wd:Q27971968 . ?item wdt:P625 ?wdcoord . 
  # now get them from Wikidata with coordinates
  BIND(geof:distance(?parlcoord, ?wdcoord) as ?dist) . filter (?dist >= 10)
  # now find out the distance (in kms)
  ?item p:P625 ?statementnode. ?statementnode psv:P625 ?valuenode.
  ?valuenode wikibase:geoLatitude ?wikilat . ?valuenode wikibase:geoLongitude ?wikilon.
  BIND(CONCAT('LINESTRING (', STR(?wikilon), ' ', STR(?wikilat), ',', STR(?long), ' ', STR(?lat), ')') AS ?str) .
  BIND(STRDT(?str, geo:wktLiteral) AS ?line) 
}

Try it!

SPARQL Federation endpoints

Not all endpoints are allowed by Wikidata Query Service. See the list of allowed endpoints on SPARQL Federation endpoints.

You can nominate more endpoints at the Wikidata:SPARQL federation input.

References

Expressions and Functions

SPARQL
Federated query

Subqueries