Property Path

Properties#

• Sequence = hidden intermediate variable. ?s p1/p2 ?o matches the same $(s, o)$ as the two triples ?s p1 ?x . ?x p2 ?o, except that the intermediate node ?x gets no column in the result.
• * has no fixed-size equivalent. A path of unbounded length can’t be written as a fixed number of triples, so * is needed for transitive relations such as “subclass of”.
• Duplicates and DISTINCT. The lecture’s query yields one result per path. If a city is reachable from wd:Q515 via several paths, it appears several times, hence SELECT DISTINCT.
• Typical Wikidata use. wdt:P31/wdt:P279* (instance of, then any number of subclass-of edges) finds all instances of a class including instances of its subclasses (Wikidata Prefix System).
• Property paths also save typing for plain chains, e.g. ?building geo:hasGeometry/geo:asWKT ?wkt in the Spatial Join example.

Example#

Toy knowledge graph:

<Freiburg> <instance_of> <University_Town> .
<University_Town> <subclass_of> <Town> .
<Town> <subclass_of> <Settlement> .
<Ulm> <instance_of> <Town> .

SELECT ?x WHERE { ?x <instance_of>/<subclass_of>* <Settlement> . }

• Freiburg: instance_of to University_Town, then 2 subclass_of edges.
• Ulm: instance_of to Town, then 1 subclass_of edge.

With <instance_of> <Settlement> alone, the result would be empty.

Lecture query (Wikidata, Slide 17):

SELECT DISTINCT ?city ?name ?population WHERE {
  ?city wdt:P31/wdt:P279* wd:Q515 .   # instance of (a subclass of) city
  ?city wdt:P17 wd:Q183 .             # country: Germany
  ?city wdt:P1082 ?population .
  ?city rdfs:label ?name FILTER (LANG(?name) = "de")
}
ORDER BY DESC(?population)

Sources#

• Slides 01, p. 17 (/, *, DISTINCT), p. 24 (geo:hasGeometry/geo:asWKT)
• ADIS_VL01 > Property Paths