query in sparql unknown relations

Question

I am new in learning Sparql. I am wondering is there a way to query nodes with no direct relation between them suppose I have the following triples in RDF

book - created_by - Author

writer - work - company

company - location - location

can I write query in Sparql to find the location of the authors of given paper we have only the paper info and we would like the retrieve the location

without knowing the relation between writer - work - company

company - location - location

Update: I am wondering if sparql have a feature similar to // in xquery and xpath // Selects nodes in the document from the current node that match the selection no matter where they are

Answer 1

Suppose you've got data like this (three books, three authors, two companies (two of the authors work at the same company), and two locations (one for each company)):

@prefix : <https://stackoverflow.com/q/23066695/1281433/>.

:book1 a :Book; :createdBy :author1 .
:book2 a :Book; :createdBy :author2 .
:book3 a :Book; :createdBy :author3 .

:author1 a :Author; :worksFor :company1. 
:author2 a :Author; :worksFor :company2. 
:author3 a :Author; :worksFor :company2. 

:company1 a :Company; :locatedIn :location1.
:company2 a :Company; :locatedIn :location2.

:location1 a :Location.
:location2 a :Location.

Then you can use a query like this to get the results that relate books to locations:

prefix : <https://stackoverflow.com/q/23066695/1281433/>

select ?book ?location where {
  ?book a :Book; (:|!:)* ?location .
  ?location a :Location .
}

-----------------------
| book   | location   |
=======================
| :book1 | :location1 |
| :book2 | :location2 |
| :book3 | :location2 |
-----------------------

The trick here is in using a property path that amounts to a wildcard. Every IRI is either : or it isn't, so the path :|!: matches every property. This means that the path (:|!:)* is a wildcard path. By requiring that ?book and ?location have types :Book and :Location, respectively, we've found paths from books to locations. See SPARQL: property paths without specified predicates and sparql complete tree from subject for more about property paths with wildcards.

There's a possible problem here, of course. RDF graphs are directed graphs, and anything can be related to anything else. This means that if you have a relation like :location2 :northOf :location1, then there's now a path from :book3 to :location1. That might not have been what you were expecting.