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sparqlrdfmultilingualblazegraph

What is the lang function in SPARQL?

Among the SPARQL Query Examples, there is a query for US presidents and spouses:

PREFIX wd: <http://www.wikidata.org/entity/> 
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX p: <http://www.wikidata.org/prop/>
PREFIX v: <http://www.wikidata.org/prop/statement/>

SELECT ?p ?w ?l ?wl WHERE {
  wd:Q30 p:P6/v:P6 ?p .
  ?p wdt:P26 ?w .
  OPTIONAL  {  
    ?p rdfs:label ?l filter (lang(?l) = "en") . 
  }
  OPTIONAL {
    ?w rdfs:label ?wl filter (lang(?wl) = "en"). 
  }
}

It contains function-like syntax, lang(?l), which is intended to filter by language.

How does this work? Is this a special function of Blazegraph? Or are different languages stored as normal values in RDF graphs?

How can I store data this way, so that it will be retrieved with the lang function correctly?

May I have my own function to have ND predicates, like probability(?a) > 0.8?

Solution

  • lang is a native SPARQL operator (see the standard) which returns the language tag of an RDF literal (e.g. the EN in ?p foaf:name "Robert"@EN.).

    Some SPARQL engines support the ability to implement custom functions. See for example this tutorial for Sesame and this for Blazegraph.