Querying the Semantic Web with SPARQL

Emanuele Della Valle [email_address] http://emanueledellavalle.org Querying the Semantic Web with SPARQL

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SPARQL in a nutshell What is SPARQL? SPARQL is the query language of the Semantic Web stays for S PARQL P rotocol a nd R DF Q uery L anguage A Query Language ...: Find names and websites of contributors to PlanetRDF : PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT ?name ?website FROM <http://planetrdf.com/bloggers.rdf> WHERE { ?person foaf:weblog ?website ; foaf:name ?name . ?website a foaf:Document } ... and a Protocol. http://.../qps? query-lang=http://www.w3.org/TR/rdf-sparql-query/ &graph-id=http://planetrdf.com/bloggers.rdf &query=PREFIX foaf: <http://xmlns.com/foaf/0.1/...

SPARQL in a nutshell Why SPARQL? SPARQL let us Pull values from structured and semi-structured data represented in RDF Explore RDF data by querying unknown relationships Perform complex joins of disparate RDF repositories in a single query Transform RDF data from one vocabulary to another Develop higher-level cross-platform application

SPARQL in a nutshell Anatomy of a SPARQL query

SPARQL in a nutshell Triple Pattern Syntax Turtle-like: URIs, QNames, literals, convenience syntax. Adds variables to get basic patterns ?var Variable names are a subset of NCNames (no "-" or ".") E.g., simple ?s a dbpedia-owl:Drug . complex ?s a dbpedia-owl:Drug . ?s skos:subject category:Anxiolytics . ?s ?p dbpedia:Kidney . Adds OPTIONAL to cope with semi-structured nature of RDF FILTER to select solution according to some criteria UNION operator to get complex patterns

SPARQL in a nutshell Writing a Simple Query Query PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?s WHERE { ?s a dbpedia-owl:Drug . } Results (click to run query on http://dbpedia.org/isparql/ ) ?s http://dbpedia.org/resource/Budesonide http://dbpedia.org/resource/Aciclovir http://dbpedia.org/resource/Salbutamol …

SPARQL in a nutshell Writing a bit more complex query Query PREFIX skos: <http://www.w3.org/2004/02/skos/core#> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug ?category WHERE { ?drug a dbpedia-owl:Drug ; skos:subject ?category . } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug ?category http://dbpedia.org/resource/Budesonide http://dbpedia.org/resource/Category:Glucocorticoids http://dbpedia.org/resource/Budesonide http://dbpedia.org/resource/Category:Asthma http://dbpedia.org/resource/Aciclovir http://dbpedia.org/resource/Category:Antivirals http://dbpedia.org/resource/Aciclovir http://dbpedia.org/resource/Category:Purines … …

SPARQL in a nutshell Basic Patterns A basic Patter is a set of triple patterns, all of which must be matched . In this case m atches the graph means find a set of bindings such that the substitution of variables for values creates a subgraph that is in the set of triples making up the graph.

SPARQL in a nutshell Matching RDF literals – Language Tag 1/2 Query PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug WHERE { ?drug a dbpedia-owl:Drug ; rdfs:label "Budesonide" . } Results (click to run query on http://dbpedia.org/isparql/ ) 0 results!!! Explanation All RDF-S label have a language tag

SPARQL in a nutshell Matching RDF literals – Language Tag 2/2 Query PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug WHERE { ?drug a dbpedia-owl:Drug ; rdfs:label "Budesonide" @en . } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug http://dbpedia.org/resource/Budesonide

SPARQL in a nutshell Matching RDF literals – numerical values As in the case of language tags, if the literals are typed (i.e., "259.34"^^xsd:float ), they do not match if they are not given explicitly. Query PREFIX xsd: <http://www.w3.org/2001/XMLSchema#> PREFIX dbpprop: <http://dbpedia.org/property/> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug WHERE { ?drug a dbpedia-owl:Drug ; dbpprop:chemspiderid "4777" ^^xsd:integer . } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug http://dbpedia.org/resource/Propranolol

SPARQL in a nutshell RDF Term Constraints SPARQL allows restricting solutions by applying the FILTER clause. An RDF term bound to a variable appears in the results if the FILTER expression, applied to the term, evaluates to TRUE. Query PREFIX xsd: <http://www.w3.org/2001/XMLSchema#> PREFIX dbpprop: <http://dbpedia.org/property/> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug ?mp WHERE { ?drug a dbpedia-owl:Drug ; dbpprop:meltingPoint ?mp . FILTER ( ?mp < 30 ) } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug ?mp http://dbpedia.org/resource/Levorphanol 23 http://dbpedia.org/resource/Streptomycin 12 … …

SPARQL in a nutshell RDF Term Constraints – regex SPARQL FILTERs allows also restricting values of strings using the regex() Query PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug ?c WHERE { ?drug a dbpedia-owl:Drug ; rdfs:comment ?c . FILTER( regex (?c, "Asthma")) } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug ?c http://dbpedia.org/resource/Budesonide … örtlichen Behandlung von Asthma bronchiale, ... http://dbpedia.org/resource/Salbutamol … Bronchospasmolytikum bei Asthma bronchiale … … …

SPARQL in a nutshell Value Tests Notation for value comparison: <, >, =, <=, >= and != Test functions Check if a variable is bound: BOUND Check the type of resource bound: isIRI, isBLANK, isLITERAL Accessing accessories: LANG, DATATYPE Logic operators: || and && Comparing strings: REGEX, langMatches Constructor functions: bool, dbl, flt, dec, int, dT, str, IRI Extensible Value Testing E.g., FILTER ( aGeo:distance(?axLoc, ?ayLoc, ?bxLoc, ?byLoc) < 10 ) . (see http://www.w3.org/TR/rdf-sparql-query/#extensionFunctions )

SPARQL in a nutshell More Sophisticated Graph Patterns RDF is "semi structured" and has no integrity constrains SPARQL addresses this issue with Group patterns match if all subpatterns match and all constraints are satisfied In SPARQL syntax, groups are { … } OPTIONAL graph patterns accommodate the need to add information to a result but without the query failing just because some information is missing. In SPARQL syntax, OPTIONAL { … } UNION graph patterns allows to match alternatives In SPARQL syntax, { … } UNION { … }

SPARQL in a nutshell Optional Graph Patterns Query PREFIX dbpprop: <http://dbpedia.org/property/> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> SELECT ?drug ?mp WHERE { ?drug a dbpedia-owl:Drug . OPTIONAL { ?drug dbpprop:meltingPoint ?mp } } Results (click to run query on http://dbpedia.org/isparql/ ) ?drug ?mp http://dbpedia.org/resource/Budesonide http://dbpedia.org/resource/Aciclovir 256.5 … …

SPARQL in a nutshell Matching alternatives with UNION Query PREFIX dbpprop: <http://dbpedia.org/property/> PREFIX dbpedia-owl: <http://dbpedia.org/ontology/> PREFIX dbpedia: <http://dbpedia.org/resource/> SELECT ?drug WHERE { ?drug a dbpedia-owl:Drug . { ?drug dbpprop:metabolism dbpedia:Kidney } UNION { ?drug dbpprop:excretion dbpedia:Kidney } } Results (click to run query on http://dbpedia.org/isparql/ ) dbpprop:metabolism dbpprop:excretion ?drug http://dbpedia.org/resource/Doripenem http://dbpedia.org/resource/Alprazolam http://dbpedia.org/resource/Piroxicam http://dbpedia.org/resource/Amlodipine …

SPARQL in a nutshell Defining the Dataset with the FROM clause A SPARQL query may specify the dataset to be used for matching by using the FROM clause and the FROM NAMED clause Ex. PREFIX rdfs:<http://www.w3.org/2000/01/rdf-schema#> PREFIX skos: <http://www.w3.org/2004/02/skos/core#> SELECT ?label ?graph FROM <http://dbpedia.org/resource/Category:Anxiolytics> FROM NAMED <http://dbpedia.org/resource/Propranolol> WHERE { ?s rdfs:label ?label . GRAPH ?graph { ?graph skos:subject ?s . } } Results below are very different from those of the same query without FROM clauses ?label ?graph [email_address] http://dbpedia.org/resource/Propranolol

SPARQL in a nutshell A difficult to solve problem 1/3 Image you don’t know dbpedia SPARQL endpoint, you discovered using Sindice the graph http://dbpedia.org/resource/ Category:Anxiolytics and you want to get the rdfs:labels of the anxiolytics. If you write the query PREFIX rdfs:<http://www.w3.org/2000/01/rdf-schema#> PREFIX skos: <http://www.w3.org/2004/02/skos/core#> PREFIX dbp-cat: <http://dbpedia.org/resource/Category:> SELECT ?label FROM <http://dbpedia.org/resource/Category:Anxiolytics> WHERE { ?s skos:subject dbp-cat:Anxiolytics . ?s rdfs:label ?label . } You get no results because the labels are in different graphs such as http://dbpedia.org/resource/Alprazolam […] http://dbpedia.org/resource/ZK-93423

SPARQL in a nutshell A difficult to solve problem 2/3 A solution is writing the query PREFIX rdfs:<http://www.w3.org/2000/01/rdf-schema#> PREFIX skos: <http://www.w3.org/2004/02/skos/core#> PREFIX dbp-cat: <http://dbpedia.org/resource/Category:> SELECT ?label FROM <http://dbpedia.org/resource/Category:Anxiolytics> FROM <http://dbpedia.org/resource/Alprazolam> FROM […] FROM <http://dbpedia.org/resource/ZK-93423> WHERE { ?s skos:subject dbp-cat:Anxiolytics . ?s rdfs:label ?label . } But this means knowing the answer already

SPARQL in a nutshell A difficult to solve problem 3/3 The Semantic Web Client provides a solution to the problem by dynamically retrieving information from the Semantic Web It dereferences HTTP URIs, It follows rdfs:seeAlso links, and it queries the Sindice search engine. Querying the Semantic Web with the following query … PREFIX rdfs:<http://www.w3.org/2000/01/rdf-schema#> PREFIX skos: <http://www.w3.org/2004/02/skos/core#> PREFIX dbp-cat: <http://dbpedia.org/resource/Category:> SELECT ?label WHERE { ?s skos:subject dbp-cat:Anxiolytics . ?s rdfs:label ?label . } … gives the correct results even if no FROM clause is given and no specific SPARQL endpoint is used.

SPARQL in a nutshell Result Forms Besides selecting tables of values, SPARQL allows three other types of queries: ASK - returns a boolean answering, does the query have any results? CONSTRUCT - uses variable bindings to return new RDF triples DESCRIBE - returns server-determined RDF about the queried resources SELECT and ASK results can be returned as XML or JSON. CONSTRUCT and DESCRIBE results can be returned via any RDF serialization (e.g. RDF/XML or Turtle).

SPARQL in a nutshell SPARQL Resources SPARQL Frequently Asked Questions http://thefigtrees.net/lee/sw/sparql-faq SPARQL implementations - community maintained list of open-source and commercial SPARQL engines http://esw.w3.org/topic/SparqlImplementations Public SPARQL endpoints - community maintained list http://esw.w3.org/topic/SparqlEndpoints SPARQL extensions - collection of SPARQL extensions implemented in various SPARQL engines http://esw.w3.org/topic/SPARQL/Extensions

Querying the Semantic Web with SPARQL

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