View source for Lab: Wikidata in RDF

==Topics==
Wikidata in RDF: 
* retrieve ''truthy'' triples about a Wikidata entity
* retrieve semantic metadata (such as qualifications and references) about the triples
* load the semantic data and metadata into GraphDB
* visualise the semantic data and metadata

''Motivation:'' So far you have built your own knowledge graph and worked on a small grap you were given. This week we will look at how to retrieve knowledge graphs from Wikidata, which can then be merged with your own graph to provide additional context. This is not a trivial problem because Wikidata most likely contains a lot more data - and in particular metadata - than you need. 

==Useful materials==
* [https://www.wikidata.org/wiki/Wikidata:SPARQL_query_service/A_gentle_introduction_to_the_Wikidata_Query_Service A gentle introduction to the Wikidata Query Service] (Simple)
* [https://www.mediawiki.org/wiki/Wikibase/Indexing/RDF_Dump_Format RDF Dump Format]

==Tasks==
'''Getting ready:'''
In a web browser, go to [http://query.wikidata.org Wikidata's Query Service (WDQS)]. Be careful to '''always use a limit like LIMIT 100 when you test things'''. Otherwise, you risk being blocked from the query service or, worse, you risk blocking out a whole subdomain.

'''Emergency data:''' 
If Wikdata's Query Service is unavailable, you can load [:File:Q42-extended.txt | this Turtle file] into GraphDB instead, and continue there using ''Q42'' as your example entity. (Remember to rename it from ''.txt'' to ''.ttl''.)

'''Task:'''
From [https://www.wikidata.org/ Wikidata's ordinary UI], find the Q-code of one of the people or entities involved in the Mueller investigation. Use that entity as your reference in the rest of this lab. (The Q-code should look like this ''https://www.wikidata.org/entity/Q42'' or ''wd:Q42''.)

'''Task:'''
Use a DESCRIBE query to retrieve some triples about your entity (remember LIMIT 100, although it is less critical on DESCRIBE queries). 

'''Task:'''
Use a SELECT query to retrieve the first 100 triples about your entity. 

'''Task:'''
Start GraphDB on your local machine. Create a new repository (''No inference'' needed), and activate it. Write a local SELECT query that embeds a <https://query.wikidata.org/bigdata/namespace/wdq/sparql> SERVICE query to retrieve the first 100 triples about your entity ''to your local machine''. 

''Tip:'' ''wd:'' is a PREFIX for ''<http://www.wikidata.org/entity/>''.

''Tip:'' To make LIMIT work inside a SERVICE query, you have to add another SELECT inside it, like this:

 SELECT ... {  # the local query
     SERVICE ... {  # the remote service
         SELECT ... {
             ...
         } LIMIT 100  # this limit works on the remote service
     }
 }  # a limit here would work on your local service, 
    # but is not strictly necessary when you alreadu have an inner LIMIT



'''Task:'''
Change the SELECT query to an INSERT query that adds the Wikidata triples your local repository. Use a local ASK and/or SELECT query to check that the triples have actually been added.