SPARQL union not merging the results

Question

I have a sparql query which is the union of three individual queries. On the union i apply a filter. Although my individual queries return rows, but the union doesn't return any rows. I have checked that the output of individual queries have rows which satisfy the outer filter criteria.

PREFIX dcterms: <http://purl.org/dc/terms/> 
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> 
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>  
PREFIX yago: <http://dbpedia.org/class/yago/> 

SELECT * 
where{

       {
    SELECT (?LandingURI) AS ?URI ?category
        WHERE {
                        {
                         ?LandingURI rdfs:label ?term .
                         ?LandingURI (dcterms:subject|rdf:type) ?category .
                        }
                FILTER((?term ="Roger"@en))
                     }
       }
       UNION
       {
       SELECT (?redirects) AS ?URI ?category
                WHERE {

                         {
                         ?LandingURI rdfs:label ?term .
                         ?LandingURI <http://dbpedia.org/ontology/wikiPageRedirects> ?redirects .
                         ?redirects (dcterms:subject|rdf:type) ?category .
                         }          
                        FILTER((?term ="Roger"@en))         
              }
       }
       UNION
       {
       SELECT (?disambiguates) AS ?URI ?category
                WHERE {

                         {
                         ?LandingURI rdfs:label ?term .
                         ?LandingURI <http://dbpedia.org/ontology/wikiPageDisambiguates> ?disambiguates .
                         ?disambiguates (dcterms:subject|rdf:type) ?category .
                         }          
                        FILTER((?term ="Roger"@en))         
              }
       }
FILTER (?category = dbpedia-owl:TennisPlayer || 
        ?category = yago:TennisOrganisations ||
        ?category = <http://dbpedia.org/resource/Category:The_Championships,_Wimbledon>
       ) 
}

Answer 1

I don't know what software you're using to process your query, but it shouldn't accept that query at all; it's not well-formed. You can check your query at sparql.org's query validator .

The bracketing in your query makes it hard to tell exactly where the union is placed, but just in case it's not , union should appear between two group graph patterns, eg: { ... } UNION { ... } .

All that said, there's actually no need to use union (or filter ) here at all. You can do this all with property paths and values . If I understand it correctly, your query can be rewritten as:

select ?uri ?term ?category where {
  #-- acceptable values of ?category
  values ?category {
    dbpedia-owl:TennisPlayer
    yago:TennisOrganisations
    <http://dbpedia.org/resource/Category:The_Championships,_Wimbledon>
  }

  #-- acceptable values of ?term
  values ?term {
    "Roger"@en
  }

  #-- get the rdfs:label of the ?landingUri, then
  #-- follow an optional redirect or disambiguation
  #-- link to the actual uri, and then get any
  #-- dcterms:subject or rdf:type from the actual uri.
  ?landingUri rdfs:label ?term .
  ?landingUri (dbpedia-owl:wikiPageRedirects|dbpedia-owl:wikiPageDisambiguates)? ?uri
  ?uri (dcterms:subject|rdf:type) ?category .
}