How can I join climate data to cities with dbpedia? - sparql

Question

I am trying to get a dataset that gives me all the data available in a city's climate table but I'm having some trouble.
I was able to get this to work and felt pretty good about myself. When I plug this in on dbpedia's virtuoso client this gives me all the cities that dbpedia has, and all of their countries.
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?city_name ?country
WHERE { ?city rdf:type dbpedia-owl:City ;
rdfs:label ?city_name;
dbpedia-owl:country ?country
FILTER (langMatches(lang(?city_name), "EN")) .
}
Update: I have found properties that seem to give what I'm looking for (e.g. dbpedia.org/property/aprHighC) but I'm having trouble adding them to my output.
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?city_name ?country ?aprHighC
WHERE { ?city rdf:type dbpedia-owl:City .
?city rdfs:label ?city_name .
?city dbpedia-owl:country ?country
FILTER (langMatches(lang(?city_name), "EN")) .
}
Gives an error: Variable 'aprHighC' is used in the query result set but not assigned. How do I assign it?

Answer 1

For the query to get results in the second query, a city has to have a three properties: rdfs:label, dbpedia-owl:country and dbpedia-owl:climate. Your query pretty much proves that DBPedia data has cities with label and country properties, but not climate. Try the following to see just what properties are found for members of dbpedia-owl:City:
SELECT DISTINCT ?p
WHERE {
?city rdf:type dbpedia-owl:City ;
?p ?o .
}
Note that not all members of dbpedia-owl:City will have these properties, but it gives you a range of what properties are used.
Looking at it the other way, you can ask what entities use the dbpedia-owl:climate property:
SELECT ?s
WHERE {
?s dbpedia-owl:climate ?climate
}
I didn't find any, so it could be the case that the prefix is different than the one you are using? I'd suggest double-checking the property name.
Regardless, it's a good idea to use SPARQL to find what is actually in the data store. And use LIMIT to look at parts of the data without overwhelming the system.

Answer 2

The following query gives the January average daily high (°C). Adding other climate items is as simple as copying the line beginning "OPTIONAL" and changing the item and variable name from janHighC to whatever you are trying to get.
PREFIX dbpedia-owl: <http://dbpedia.org/ontology/>
SELECT * {
{ ?city rdf:type dbo:City .
?city rdf:type schema:City .
?city rdfs:label ?name
}
OPTIONAL {?city dbp:janHighC ?janHighC .}
}
I will note, however, that most cities don't have this information. I had to give up on getting the data this way.