SPARQL using subquery with limit

[Paton, Diego]

Hi,

I am developing a java application that uses ARQ to execute SPARQL queries 
using a Fuseki endpoint over TDB.

The application needs a query that returns the place of birth of each person 
and other person that was born in the same place.

To start, I wrote this SPARQL query that returns person_ids and the place of 
birth of each person.

prefix fb: <http://rdf.freebase.com/ns/>
prefix fn: <http://www.w3.org/2005/xpath-functions#>
select ?person_id ?place_of_birth 
where {
    ?person_id fb:type.object.type fb:people.person .
    ?person_id fb:people.person.place_of_birth ?place_of_birth_id .
    ?place_of_birth_id fb:type.object.name ?place_of_birth .
     FILTER (langMatches(lang(?place_of_birth),"en"))
}
LIMIT 10

----------------------------------
| person_id    | place_of_birth  |
==================================
| fb:m.01vtj38 | "El Centro"@en  |
| fb:m.01vsy7t | "Brixton"@en    |
| fb:m.09prqv  | "Pittsburgh"@en |
----------------------------------

After that, I added a subquery 
(https://jena.apache.org/documentation/query/sub-select.html) adding other 
person who was born there, but I get more than one person related and I only 
need one.


prefix fb: <http://rdf.freebase.com/ns/>
prefix fn: <http://www.w3.org/2005/xpath-functions#>
select ?person_id ?place_of_birth ?other_person_id
where {
    ?person_id fb:type.object.type fb:people.person .
    ?person_id fb:people.person.place_of_birth ?place_of_birth_id .
    ?place_of_birth_id fb:type.object.name ?place_of_birth .
    {
       select  ?other_person_id
       where {
       ?place_of_birth_id fb:location.location.people_born_here 
?other_person_id .
       }
     }
     FILTER (langMatches(lang(?place_of_birth),"en"))
}
LIMIT 10

---------------------------------------------------
| person_id    | place_of_birth | other_person_id |
===================================================
| fb:m.01vtj38 | "El Centro"@en | fb:m.01vtj38    |
| fb:m.01vtj38 | "El Centro"@en | fb:m.01vsy7t    |
| fb:m.01vtj38 | "El Centro"@en | fb:m.09prqv     |
---------------------------------------------------

I have tried to add a LIMIT 1 subquery but it seems that does not work ( the 
query is executed but never ends )

prefix fb: <http://rdf.freebase.com/ns/>
prefix fn: <http://www.w3.org/2005/xpath-functions#>
select ?person_id ?place_of_birth ?other_person_id
where {
    ?person_id fb:type.object.type fb:people.person .
    ?person_id fb:people.person.place_of_birth ?place_of_birth_id .
    ?place_of_birth_id fb:type.object.name ?place_of_birth .
    {
       select  ?other_person_id
       where {
       ?place_of_birth_id fb:location.location.people_born_here 
?other_person_id .
       }
       LIMIT 1
     }
     FILTER (langMatches(lang(?place_of_birth),"en"))
}
LIMIT 3


Therefore, is there a way to return only 1 result in the subquery ? or can't do 
that using SPARQL.

Thanks in advance,
Regards,

Diego.

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