getting city information by sparql - sparql

Question

I tried with the following SPARQL query.
SELECT distinct ?city ?cityName ?country ?population ?knownfor WHERE {
?city rdf:type dbo:City .
?city rdfs:label ?cityName.
?city dbo:country ?country.
OPTIONAL{
?city dbp:population ?population.
?city dbo:knownFor ?knownfor.
}
FILTER (lang(?cityName) = 'en')
} ORDER BY ?city
But, the problem is -
Not every city has dbp:population predicate but some city has dbp:populationTotal. So, for some cities we can get the population by this but when i write in the optional section of the query -
OPTIONAL{
?city dbp:population ?population .
?city dbp:populationTotal ?populationTotal
}
the both section become blank. Same goes for dbo:knownFor predicate (not every city have knownFor predicate).
How can I specify in the query that i want only the European city? I can not find any predicate which specify the continent of the city

Answer 1

First thing to know is that DBpedia data is a moving target, just like the Wikipedia data from which it is derived. Updates to Wikipedia will eventually be part of DBpedia. More quickly, they'll be part of DBpedia-live.
The issue with values for neither OPTIONAL predicate showing up when you include both predicates appears to be a bug in the Virtuoso version currently hosting DBpedia. I encourage you to check whether it's been reported yet, report it yourself if not, and monitor the issue.
As to limiting the continent of the cities you get back -- it's usually easiest to check an entity of (or near) the sort you want, to find a relevant attribute/predicate/property. For instance, Aachen-Mitte has a dbo:country of Germany, which has a number of rdf:types including yago:EuropeanCountries -- which might be what you want, but might not yet have been applied to all such. You'll need to add a triple to your pattern like --
?country a yago:EuropeanCountries
Edit To Add
The OPTIONAL { ... } clause returns results for the entire pattern enclosed within the braces. So --
OPTIONAL
{
?city dbp:population ?population .
?city dbo:knownFor ?knownfor .
}
-- will only return values for either predicate when that ?city has values for both predicates.
If you want to get every value for either predicate, you need to split that clause into two --
OPTIONAL
{
?city dbp:population ?population .
}
OPTIONAL
{
?city dbo:knownFor ?knownfor .
}
It's easy to get both dbp:population and dbp:populationTotal, with both OPTIONAL (and adding ?populationTotal to your SELECT list) --
SELECT DISTINCT ?city
?cityName
?country
?population
?populationTotal
?knownfor
...
OPTIONAL
{
?city dbp:population ?population .
}
OPTIONAL
{
?city dbp:populationTotal ?populationTotal .
}
OPTIONAL
{
?city dbo:knownFor ?knownfor .
}
If you only want one of the population values, and especially if you have a preference of one predicate over the other, the construction gets more complex (and should be a new question).