Wikidata query - how to add year to city's population? - sparql

Question

I would like to get the year of population count for each city.
Do tou know how to add it correctly? Currently I got empty results.
Here's my query:
SELECT DISTINCT ?cityLabel ?population ?gps ?data WHERE {
?city (wdt:P31/(wdt:P279*)) wd:Q515;
wdt:P1082 ?population;
wdt:P625 ?gps.
OPTIONAL { ?population wdt:P585 ?date. } # here I have a problem
SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}
ORDER BY DESC(?population) LIMIT 100
PS. just paste it here: https://query.wikidata.org/

Answer 1

First problem: you are selecting ?data but the actual variable is ?date.
Second problem: ?population is the object of your statement, but qualifiers refer to a whole statement, not just its object.
For referring to the statement, you'll have to use p:P1082 instead of wdt:P1082.
You can obtain what you want with the following query:
SELECT DISTINCT ?cityLabel ?population ?gps ?date WHERE {
?city
wdt:P31/wdt:P279* wd:Q515;
wdt:P625 ?gps.
?city p:P1082 ?populationStatement .
?populationStatement ps:P1082 ?population .
OPTIONAL { ?populationStatement pq:P585 ?date. }
SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}
ORDER BY DESC(?population)
LIMIT 10
I set LIMIT 10 because this is a pretty heavy query and sometimes it reaches timeout.
To deepen the topic, I'd suggest you to read Wikidata:SPARQL tutorial§Qualifiers.