What is SPARQL?

A knowledge graph is only useful if you can ask it questions. SPARQL handles the questions that are awkward in SQL: following chains of relationships, matching patterns across data from different sources, and combining results from several databases.

A SPARQL query is declarative. You describe the pattern you are looking for and the triplestore finds every match. A question like “find all subsidiaries of EU-headquartered companies that filed a patent in the last three years” is one short pattern in SPARQL, where SQL would need several joins and subqueries.

SPARQL can also federate: one query can read from several triplestores. That lets an organisation combine its own data with external linked data sources without copying them in first.

A SPARQL query is built around a graph pattern, a set of triple patterns with variables that the engine matches against the data. A basic example:

SELECT ?company ?ceo
WHERE {
  ?company a ex:Corporation .
  ?company ex:ceo ?ceo .
  ?company ex:headquarters ex:London .
}

This finds every corporation headquartered in London and returns its CEO. SPARQL has four query forms: SELECT returns a table, CONSTRUCT returns new RDF, ASK returns true or false, and DESCRIBE returns what is known about a resource.

Beyond the basics there are OPTIONAL patterns (similar to a left join), FILTER expressions, GROUP BY and HAVING for aggregation, property paths for following chains of any length, and SERVICE for sending part of a query to a remote endpoint.

Wikidata Query Service. Anyone can run SPARQL against Wikidata’s billions of triples, to answer questions such as “which Nobel Prize winners were born in cities with more than a million people?”

Financial compliance. A bank can use SPARQL to walk its beneficial ownership graph and answer a regulator’s question like “show every entity with more than 25% ownership that is registered in a high-risk jurisdiction”.

Drug discovery. Researchers can query a biomedical knowledge graph for every protein linked to a disease pathway that has a known small-molecule inhibitor, which narrows down the targets worth investigating.