This page provides various explanations that are useful when working the Wikidata and its peculiar schema.

A good SPARQL tutorial specifically for Wikidata and its particular schema can be found here: https://www.wikidata.org/wiki/Wikidata:SPARQL_tutorial

Standard prefixes can be found here

Ranking entities by the number of sitelinks or the number of triples

A sitelink is a Wikipedia page that is about a certain Wikidata entity. The number of sitelinks is a good proxy for the popularity of an entity. For example, to get all German cities and their population ordered by the number of sitelinks, one can write the following. If you run the query on the Wikidata Query Service (which only works with a relatively small LIMIT), note how in the top results there is strong correlation to population size.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX schema: <http://schema.org/>
PREFIX wikibase: <http://wikiba.se/ontology#>
SELECT ?label ?population ?sitelinks WHERE {
  ?city wdt:P31 wd:Q515 .          # ?city "instance of" "city" 
  ?city wdt:P17 wd:Q183 .          # ?city "country" "Germany" 
  ?city wdt:P1082 ?population .    # ?city "population" ?population
  ?city rdfs:label ?label .  
  ?city ^schema:about/wikibase:sitelinks ?sitelinks .
  FILTER (LANG(?label) = "en")
}
ORDER BY DESC(?sitelinks)

An alternative measure is the number of statements of a particular entity. This often gives unintuitive results, because there are many entities with a large number of "artificial" triples (for example, all kinds of IDs), without necessarily being particulary popular.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX schema: <http://schema.org/>
PREFIX wikibase: <http://wikiba.se/ontology#>
SELECT ?label ?population ?statements WHERE {
  ?city wdt:P31 wd:Q515 .          # ?city "instance of" "city" 
  ?city wdt:P17 wd:Q183 .          # ?city "country" "Germany" 
  ?city wdt:P1082 ?population .    # ?city "population" ?population
  ?city rdfs:label ?label .  
  ?city ^schema:about/wikibase:statements ?statements .
  FILTER (LANG(?label) = "en")
}
ORDER BY DESC(?statements)

Note that the schema:about and wikibase:sitelinks and wikibase:statements information requires the full Wikidata dataset. These predicates are not part of the truthyversion. Also note that the dot before the FILTER clause is not necessary according to the SPARQL standard, but QLever currently (07.02.2020) still requires it.

Ranking by number of triples

A simpler way to rank entities and which also works with the truthy data, is to order entities by the number of triples in which they are used as subject. This works well for some queries (like the following), but not so well for others (because there are quite a lot of relatively "unimportant" entities, but which have a lot of "phony" triples; TODO: give an example).

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX schema: <http://schema.org/>
PREFIX wikibase: <http://wikiba.se/ontology#>
SELECT ?label ?population (COUNT(?label) AS ?popularity) WHERE {
  ?city wdt:P31 wd:Q515 .
  ?city wdt:P17 wd:Q183 .
  ?city wdt:P1082 ?population .
  ?city rdfs:label ?label .
  ?city ?p ?o .  
  FILTER (LANG(?label) = "en")
}
GROUP BY ?label ?population
ORDER BY DESC(?popularity)
LIMIT 100

Obtaining names

Names for entities

The Wikidata Query Service (WDQS) has an automatic mechanism for obtaining the single best name for entity variables in the query. It does not work for predicate names, however. The mechanism is invoked by adding this line to the SPARQL query:

SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }

To obtain a single name of an entity "manually" (= without the mechanism above), one can add the following two lines to a query (Q90 is Paris). Note that without the language filter, one gets one name for each languages, for which a name has been specified for that entity in Wikidata (for popular entities, these are usually very many):

wd:Q90 rdfs:label ?label .
FILTER (LANG(?label) = "en") 

Names for predicates

Obtaining the name for a predicate is a bit more complicated. Here is how to obtain all predicates and their English name for an entity (again Q90), sorted by the number of triples with that predicate and that entity as subject:

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?p (COUNT(?o) as ?count) (SAMPLE(?label) as ?pname) WHERE {
  wd:Q90 ?p ?o .
  ?x wikibase:claim ?p .
  ?x rdfs:label ?label .
  FILTER (LANG(?label) = "en")
}
GROUP BY ?p
ORDER BY DESC(?count)
LIMIT 100

There are a number of other wikibase: predicates that relate the different types of predicates to teach other

wd:P47 wikibase:claim              p:P47
wd:P47 wikibase:directClaim        wdt:P47
wd:P47 wikibase:novalue            wdno:P47
wd:P47 wikibase:qualifier          pq:P47
wd:P47 wikibase:qualifierValue     pqv:P47
wd:P47 wikibase:reference          pr:P47
wd:P47 wikibase:referenceValue     prv:P47
wd:P47 wikibase:statementProperty  ps:P47
wd:P47 wikibase:statementValue     psv:P47

Aliases of an entity

In the .nt file, Aliases are expressed as follows

<http://www.wikidata.org/entity/Q567> <http://www.w3.org/2004/02/skos/core#altLabel> "Angela Dorothea Merkel"@de

There are also some other predicates, which provide alternative names, see this question on Stackoverflow. To get all aliases of an entity according to that list write something like this:

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?alias WHERE {
  wd:Q567 (wdt:P735|wdt:P734)?/(wdt:1449|wdt:P742|skos:altLabel|rdfs:label) ?alias .
  FILTER (lang(?alias) = "en")
}

The predicates wdt:P735 and wdt::P734 yield the given name and family name, respectively. Note that the objects of the predicates are entities, not literals.

The ? after the (...) means that sequences of length 0 or 1 of these predicates are considered (that is using one of the predicates or not). With * one would get all sequences of length >= 0. For the query above, this would give the same result. With + one would get all sequences of length >= 1. For the query above, this would give fewer results, because we would not get the names of only wd:Q567.

The predicates wdt:1449 and skos:altLabel and rdfs:label yield the nicknames and alternative names and unique (per language) label, respectively. The objects for these predicates are literals. Note that the literals from skos:altLabel are in general not a superset of the literals from rdfs:label.

Note that skos stands for simple knowledge organisation system. The simple is obviously a joke, given that the full URI is http://www.w3.org/2004/02/skos/core#altLabel .

Statements and value-qualifier tuples for an entity

For example, all (population, point in time) pairs for Paris (Q90):

PREFIX p: <http://www.wikidata.org/prop/>
PREFIX ps: <http://www.wikidata.org/prop/statement/>
PREFIX pq: <http://www.wikidata.org/prop/qualifier/>
PREFIX wd: <http://www.wikidata.org/entity/>
SELECT ?value ?time WHERE {
  wd:Q90 p:P1082 ?statement . # P1082 = population
  ?statement ps:P1082 ?value . # P1082 = population
  ?statement pq:P585 ?time # P585 = point in time
}
ORDER BY DESC(?time)

Problems with the Wikidata schema

Query: City, country, population

1. Many cities in Wikidata are not "instance of" (wdt:P31) "city" (wd:Q515), but "big city" (wd:Q4905452), which in turn is a "subclass of" (wdt:P279) city. Many of the example queries in Wikidata therefore use the following triple to specify that a variable is a city:

?city wdt:P31/wdt:P279* wd:Q515

2. Unlike Freebase, Wikidata has a property "country" (wdt:P17) to relate items (in particular: cities) to their countries. There is also "contains administrative territorial entity" (wdt:P150), but it is much less complete: the first query below has a result with 10,560,170 rows, the second query has a result with only 21,344 rows.

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
SELECT ?city ?country WHERE {
  ?city wdt:P17 ?country .
}

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wd: <http://www.wikidata.org/entity/>
SELECT ?city ?country {
  ?city wdt:P31/wdt:P279* wd:Q515 .
  ?country wdt:P31 wd:Q6256 .
  ?city wdt:P17 ?country
}