Length of path between nodes, allowing for multiple relation types (SPARQL) - sparql

Question

I have to write a SPARQL query that returns the length of the path between two nodes (:persA and :persD) that are connected by these relationships:
#prefix : <http://www.example.org/> .
:persA :knows :knowRelation1 .
:knowRelation1 :hasPerson :persB .
:persB :knows :knowRelation2 .
:knowRelation2 :hasPerson :persC .
:persC :knows :knowRelation3 .
:knowRelation3 :hasPerson :persD .
I tried with this query:
PREFIX : <http://www.example.org/>
SELECT (COUNT(?mid) AS ?length)
WHERE
{
:persA (:knows | :hasPerson)* ?mid .
?mid (:knows | :hasPerson)+ :persD .
}
the result seems to be a infinite loop.
Any advice/examples of how this can be done?

Answer 1

After fixing some syntax errors in your initial post, the provided triples and query work for me in GraphDB Free 8.2 and BlazeGraph 2.1.1. I have since applied these edits to your post itself.
added a trailing / to your definition of the empty prefix
added a trailing . to your prefix definition line (required if you want to start with a #)
fixed the spelling of length (OK, that's just a cosmetic fix)
.
#prefix : <http://www.example.org/> .
:persA :knows :knowRelation1 .
:knowRelation1 :hasPerson :persB .
:persB :knows :knowRelation2 .
:knowRelation2 :hasPerson :persC .
:persC :knows :knowRelation3 .
:knowRelation3 :hasPerson :persD .
.
PREFIX : <http://www.example.org/>
SELECT (COUNT(?mid) AS ?length)
WHERE
{ :persA (:knows|:hasPerson)* ?mid .
?mid (:knows|:hasPerson)+ :persD
}
result:
length
"6"^^xsd:integer