Property paths
The query construction that allows us to select items that belong to the same class makes use of property paths. Property paths are shorthand for writing down a path of properties between two items.
To understand how this construction works, take a look at the graphic view of some information about the item Star Wars: Episode I – The Phantom Menace (Q165713):
Item Q165713 has a P31 (instance of) statement with “feature film” (Q24869) as its value. So the Item-Property-Value statement would be:
Q165713 – P31 – Q24869
The path between Q165713 and Q24869 is the simplest path: a single property.
Item Q24869 (feature film) has the property P279 (subclass of) with the value Q11424 (film). So the Item-Property-Value statement would be:
Q24869 – P279 – Q11424
The path between Q24869 and Q11424 is also just a single property.
Path elements can be put together with a forward slash (/). So a query statement that uses the construction wdt:P31/wdt:P279 denotes a property path between two items consisting of P31 (instance of) and P279 (subclass of).
However, if our pattern for matching would be:
?item wdt:P31/wdt:P279 wd:Q11424.
the query would match only items that are an instance of a subclass of film, meaning only items that have a path consisting of P31 and P279 to the item film (Q11424).
The construction wdt:P31/wdt:P279* on line 6 is shorthand for saying that there’s an “instance of” property and then any number of “subclass of” properties between ?item and the item “film” (Q11424).
If you remove the asterisk (*) on line 6 of the query above and run the query again you will see that the query does not retrieve those items that are themselves an instance of film (Q11424).
The asterisk (*) after the path element means “zero or more of this element”. Thus the selection pattern
?item wdt:P31/wdt:P279* wd:Q11424
could match:
?item wdt:P31 wd:Q11424.
or
?item wdt:P31/wdt:P279 wd:Q11424.
or
?item wdt:P31/wdt:P279/wdt:P279 wd:Q11424.
or
?item wdt:P31/wdt:P279/wdt:P279/wdt:P279 wd:Q11424.
and so on.