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20a2ece May 13, 2015
@smalyshev @earldouglas
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Exploring linked data

In the following examples, we'll see how we can discover information by exploring linked data by walking through a few SPARQL analogs of "casually clicking around the Web".

George Washington's linked data

George Washington has the identifier Q23 on Wikidata. Let's see what we can learn about the national founder through RDF.

PREFIX entity: <http://www.wikidata.org/entity/>

SELECT ?predicate ?object WHERE {
  entity:Q23 ?predicate ?object .
} LIMIT 10

This query asks for the predicate and object of every subject/predicate/object triple which has the subject Q23. It yields the following:

predicate object
schema:dateModified 2015-04-23T01:45:55Z
schema:description American politician, 1st president of the United States (in office from 1789 to 1797)
schema:version 212398883
http://www.wikidata.org/prop/direct/P1005 64839
http://www.wikidata.org/prop/direct/P1006 069038171
http://www.wikidata.org/prop/direct/P1017 ADV10077200
http://www.wikidata.org/prop/direct/P106 http://www.wikidata.org/entity/Q131512
http://www.wikidata.org/prop/direct/P106 http://www.wikidata.org/entity/Q1734662
http://www.wikidata.org/prop/direct/P106 http://www.wikidata.org/entity/Q189290

Let's explore the statement in last row:

<http://www.wikidata.org/entity/Q23> <http://www.wikidata.org/prop/direct/P106> <http://www.wikidata.org/entity/Q189290> .

On Wikidata, P106 is the identifier for the occupation property, and Q189290 is the identifier for the (military) officer entity. In other words, this statement says that George Washington held the occupation of officer. This is interesting, but it doesn't tell us a whole lot about where or where Washington was an officer, or what rank he held during that time.

Let's find out everything we know about this item.

PREFIX entity: <http://www.wikidata.org/entity/>
PREFIX p:      <http://www.wikidata.org/prop/>
PREFIX ps:     <http://www.wikidata.org/prop/statement/>

SELECT ?predicate ?object WHERE {
  entity:Q23 p:P106 ?subject .
  ?subject ps:P106 entity:Q189290 .
  ?subject ?predicate ?object .
} LIMIT 10
predicate object
http://www.wikidata.org/prop/statement/P106 http://www.wikidata.org/entity/Q189290
http://www.wikiba.se/ontology#rank http://www.wikiba.se/ontology#BestRank
http://www.wikiba.se/ontology#rank http://www.wikiba.se/ontology#NormalRank

There's nothing very interesting here; it looks like a dead end. Let's explore a different path, substituting position held (P39) for occupation. First, we'll check whether Washington held any positions:

PREFIX entity: <http://www.wikidata.org/entity/>
PREFIX a:      <http://www.wikidata.org/prop/direct/>

SELECT ?object WHERE {
  entity:Q23 a:P39 ?object .
} LIMIT 10
object
http://www.wikidata.org/entity/Q11696
http://www.wikidata.org/entity/Q1115127

It looks like Washington held the position with the Wikidata identifier Q11696, or President of the United States of America. Let's find out more about it.

PREFIX entity: <http://www.wikidata.org/entity/>
PREFIX p:      <http://www.wikidata.org/prop/>
PREFIX ps:     <http://www.wikidata.org/prop/statement/>

SELECT ?predicate ?object WHERE {
  entity:Q23 p:P39 ?subject .
  ?subject ps:P39 entity:Q11696 .
  ?subject ?predicate ?object .
} LIMIT 10
predicate object
http://www.w3.org/ns/prov#wasDerivedFrom http://www.wikidata.org/reference/7eb64cf9621d34c54fd4bd040ed4b61a88c4a1a0
http://www.wikidata.org/prop/qualifier/P1365 http://www.wikiba.se/ontology#Novalue
http://www.wikidata.org/prop/qualifier/P1366 http://www.wikidata.org/entity/Q11806
http://www.wikidata.org/prop/qualifier/P580 1789-04-30T00:00:00Z
<http://www.wikidata.org/prop/qualifier/value/P580 http://www.wikidata.org/value/75cbf81427a9b5854184f36402952925
http://www.wikidata.org/prop/qualifier/P582 1797-03-04T00:00:00Z
http://www.wikidata.org/prop/qualifier/value/P582 http://www.wikidata.org/value/2b259e264018fbb2123f2667d6912f0e
http://www.wikidata.org/prop/statement/P39 http://www.wikidata.org/entity/Q11696
http://www.wikiba.se/ontology#rank http://www.wikiba.se/ontology#BestRank
http://www.wikiba.se/ontology#rank http://www.wikiba.se/ontology#NormalRank

There's lots of information about Washington's presidency. We can see a couple of dates with the years 1789 and 1797, which seem likely to be the beginning and ending of his presidency. Checking Wikidata, we can see that the corresponding property identifiers, P580 and P582, are indeed start time and end time, respectively.

Data linked to George Washington

Let's flip things around and see what data are associated with George Washington.

PREFIX entity: <http://www.wikidata.org/entity/>

SELECT ?subject ?predicate WHERE {
  ?subject ?predicate entity:Q23 .
} LIMIT 10
subject predicate
http://www.wikidata.org/entity/Q191789 http://www.wikidata.org/prop/direct/P26
http://www.wikidata.org/entity/Q458119 http://www.wikidata.org/prop/direct/P40
http://www.wikidata.org/entity/Q5083373 http://www.wikidata.org/prop/direct/P7
http://www.wikidata.org/entity/Q7412891 http://www.wikidata.org/prop/direct/P7
http://www.wikidata.org/entity/Q8488276 http://www.wikidata.org/prop/direct/P301
http://www.wikidata.org/entity/Q850421 http://www.wikidata.org/prop/direct/P7
http://www.wikidata.org/entity/Q61 http://www.wikidata.org/prop/direct/P138
http://www.wikidata.org/entity/Q1223 http://www.wikidata.org/prop/direct/P138
http://www.wikidata.org/entity/Q511164 http://www.wikidata.org/prop/direct/P138
http://www.wikidata.org/entity/statement/q511164-cd0598a2-4b84-3acf-79c0-5c044cb6a054 http://www.wikidata.org/prop/statement/P138

A bunch of entities are linked to Washington via a variety of properties. Let's find out about the first one:

PREFIX entity: <http://www.wikidata.org/entity/>

SELECT ?subject WHERE {
  entity:Q191789 rdfs:label ?subject .
} LIMIT 10
subject
Martha Washington

We found Washington's wife. She is associated with him via P26, the Wikidata identifier for spouse.

To discover this without manually looking it on the Wikidata wiki, we can inspect the label of the entity that connects Washington's wife to him:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX entity: <http://www.wikidata.org/entity/>
PREFIX wikibase: <http://wikiba.se/ontology#>

SELECT ?relationship WHERE {
  entity:Q191789 ?predicate entity:Q23 .
  ?property wikibase:directClaim ?predicate .
  ?property rdfs:label ?relationship .
} LIMIT 10
relationship
spouse