Obtaining the rdf:type of the root node

Question

I have the following data:

@prefix ex: <http://example.com/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix sch: <http://schema.org/> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

<ex:a4fe06ac906b092870> a ex:XXXX ;
    ex:pA [ a ex:YYYY ;
            ex:bar "value" ;
            ex:foo "" ;
            ex:name "another_name" ] ;
    ex:pB "another_value" ;
    ex:pC [ a ex:ZZZZ ;
            ex:identifier "the id" ;
            ex:index "asdf" ] .

which can be visualized as this.

The root node is defined to be the node with no incoming edges.

The root node in this sample is ex:a4fe06ac906b092870 and the rdf:type is ex:XXXX.

What SPARQL query would return to me ex:XXXX?

if the "root node" means no incoming edges you could do `select ?type { ?s a ?type filter not exists {?s_in ?p_in ?s} }` otherwise, you have to define the distinguishing feature(s) that make a node being a root node and then try to map those to SPARQL. Clearly, since RDF can be used to model (pseudo) graphs which can contain cycles, or the data can have multiple "roots", or ... compared to trees, SPARQL has some obvious limitations — UninformedUser, Apr 28 '20 at 19:04

score 2 · Accepted Answer · answered Apr 28 '20 at 19:54

Based on UninformedUser's comment, the SPARQL query that will return the rdf:type of the root node is:

        SELECT ?o 
        WHERE { 
            ?s a ?o . 
            FILTER NOT EXISTS {
                ?s_in ?p_in ?s .
            } 
        }

Since the rdf:type of the root node is desired, only those subjects with a rdf:type edge need to be considered ( ?s a ?o . ).

?s_in ?p_in ?s . in the filter will match all of the nodes with a rdf:type edge which have an incoming edge. These nodes are then filtered out of the results.

What is left is the rdf:type of the root node.

Obtaining the rdf:type of the root node

1 Answers1