SPARQL query does not give all the results that are on dbpedia - sparql

Question

I'm executing this query in http://dbpedia.org/snorql/:
Query:
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX type: <http://dbpedia.org/class/yago/>
PREFIX prop: <http://dbpedia.org/property/>
SELECT ?country_name ?population
WHERE {
?country a type:LandlockedCountries ;
rdfs:label ?country_name ;
prop:populationEstimate ?population .
}
The query looks for all land looked countries.
I don't understand why in the results there are not some countries which are classified on
"/dbpedia.org/class/yago/LandlockedCountries". For example, Paraguay (/dbpedia.org/page/ParaguAy) is classified but does not appear in the query result set. Can somebody explain me why?

Answer 1

Unfortunately, there are a small number of landlocked countries that do not have values for at least one of the country_name and populationEstimate properties. This is why they will not be returned in your query. If you run the following query, those countries will come up (those two attributes are set as OPTIONAL).
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX type: <http://dbpedia.org/class/yago/>
PREFIX prop: <http://dbpedia.org/property/>
SELECT ?country ?country_name ?population
WHERE {
?country a type:LandlockedCountries .
OPTIONAL {?country rdfs:label ?country_name Filter(lang(?country_name) = 'en')} .
OPTIONAL {?country prop:populationEstimate ?population} .
}
run query
For (slightly) better results, since some countries seem to be duplicated with erroneous capitalization (e.g. ParaguAy and Paraguay), the following query uses ?country dcterms:subject category:Landlocked_countries instead of the yago class.
run query