You can use limits with subqueries
You can use limits in subqueries. Here's an example:
select ?x ?y where {
values ?x { 1 2 3 4 }
{
select ?y where {
values ?y { 5 6 7 8 }
}
limit 2
}
}
limit 5
---------
| x | y |
=========
| 1 | 5 |
| 1 | 6 |
| 2 | 5 |
| 2 | 6 |
| 3 | 5 |
---------
As you can see, you get two values from the subquery (5 and 6), and these are combined with the bindings from the outer query, from which we get five rows in total (because of the limit).
Subqueries are evaluated innermost first
However, keep in mind that subqueries are evaluated from the innermost first, to the outermost. That means that in your query,
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
you are finding one match for
?place_of_birth_id fb:location.location.people_born_here ?other_person_id .
and passing the ?other_person_id binding out into the outer query. The rest of the outer query doesn't use ?other_person_id, though, so it doesn't really have any effect on the results.
What to do instead
If you need only one other person
The application needs a query that returns the place of birth of each person and other person that was born in the same place.
Conceptually, you could look at this as picking a person, finding their place of birth, and sampling one more person from the people born in that place. You can actually write the query like that, too:
select ?person_id ?place_of_birth (sample(?other_person_idx) as ?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 .
FILTER (langMatches(lang(?place_of_birth),"en"))
?place_of_birth_id fb:location.location.people_born_here ?other_person_idx .
filter ( ?other_person_idx != ?person_id )
}
group by ?person_id ?place_of_birth
If you need more than one
This is a much trickier problem if you need more than one "other result" for each result. That's the problem in Nested queries in sparql with limits. There's an approach in How to limit SPARQL solution group size? that can be used for this.
