Multiple SELECT query for getting all children of a given root - sparql

Question

I want to define the root categories corresponding to interests of users. Then I need to return all other potential interests under given root directory.
I tried the following query, but it looks like it enters into a loop (the query is executing internally).
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX owl: <http://www.myweb.com/myontology.owl#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?user ?othercat
WHERE
{
?othercat rdfs:subClassOf ?root .
{
SELECT ?user ?retailcat ?root
WHERE {
?user rdf:type owl:User .
?user owl:hasUserProfile ?userprofile .
?userprofile rdf:type owl:UserProfile .
?userprofile owl:interestedIn ?retailcat .
?entity rdf:type ?type .
?type rdfs:subClassOf* ?retailcat .
?retailcat rdfs:subClassOf ?root .
}
}
}
Indeed when I execute the sub-query, it works fine, but it returns current interests of a user, without providing the information of other child-concepts of the same root.
How to solve this issue?

Answer 1

I tried the following query, but it looks like it enters into a loop (the query is executing internally).
There shouldn't be a way for a SPARQL query to enter into a loop. Subqueries are executed first, and then the enclosing query is executed. There's no way to re-execute the inner part or anything like that. (Of course, a query could be expensive and take a long time, but that's not a loop, and is still bounded, in principle.)
As an aside, using owl: as prefix for something other than the standard OWL namespace is somewhat confusing, and likely to mislead other developers when they see your query. There's nothing incorrect about it per se, but you might want to consider using a different prefix for your namespace.
You do have one part of your query that could make things rather expensive. You have the pattern
?entity rdf:type ?type .
?type rdfs:subClassOf* ?retailcat .
where ?entity isn't connected to anything else, and isn't used anywhere else. That means that you'll have a subquery solution for every possible value of ?entity, and that just means you're multiplying the number of results by the number of possible values of ?entity.
If I understand your query, you're trying to go from a user's categories of interest, up the category tree to some root concepts and then find other categories under that root. You don't actually need a subquery for that; you can do it with a non-nested query:
select ?user ?othercat {
?user rdf:type owl:User .
?user owl:hasUserProfile ?userprofile .
?userprofile rdf:type owl:UserProfile .
?userprofile owl:interestedIn ?retailcat .
?retailcat rdfs:subClassOf ?root .
?othercat rdfs:subClassOf ?root .
}
That will find values of ?othercat that are siblings of ?retailcat, along with ?retailcat itself. If you want to avoid ?retailcat, you can add
filter(?othercat != ?retailcat)
but that shouldn't really impact performance much; that's just one result to filter out.
The only other factor that you might want to consider is that you're not really finding a "root" of the category tree with rdfs:subClassOf; you're just going up one level. E.g., if your category tree looks like
Automobile
SUV
SportsCar
Corvette
Mustang
and a user is interested in Mustang, then you'll go up to SportsCar and find Corvette, but you won't be going any farther up the tree. (If you have inference available, you may actually go farther up the tree, but I'm assuming for the moment that you don't.) To follow the subclass links up the tree, you can add * to the path to follow the chain:
?retailcat rdfs:subClassOf* ?root .
?othercat rdfs:subClassOf ?root .
Then you'd get all the classes in the tree (except the very top level one, Automobile).