Day 2. RISIS Linked Data Course

Transcription

1 Day 2 RISIS Linked Data Course

2 Overview of the Course: Friday 9:00-9:15 Coffee 9:15-9:45 Introduction & Reflection 10:30-11:30 SPARQL Query Language 11:30-11:45 Coffee 11:45-12:30 SPARQL Hands-on 12:30-13:30 Lunch 13:30-14:00 Linksets & Lenses for Aligning Datasets 14:00-14:30 Linked Data Publishing 14:30-14:45 Coffee 14:45-16:00 Data Publishing Hands-On 16:00-17:00 Actual Research

3 SPARQL Query Language Rinke Hoekstra,

4 Questions Goals How can we query Linked Data Know the syntax of SPARQL What is the syntax of SPARQL How can we use SPARQL Understand how to query Linked Data on the Web Know about standard vocabularies in Linked Data

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6 Deep Breath by Tomas A

7 RDF - Where is it? As separate files, e.g. as.ttl,.rdf,.nt, etc. Integrated with Web pages (RDFa/Microdata) Accessible through content negotiation curl -L -H "Accept: text/turtle" " curl -L -H "Accept: text/html" " In RDF-specific databases called triple stores A standard HTTP REST API for querying using the SPARQL language This API is called a SPARQL endpoint

8 SPARQL - The SPARQL Protocol and RDF Query Language The SPARQL Protocol specifies how to query Triple stores over HTTP The SPARQL Query Language specifies a syntax for writing queries There are six types of queries SELECT, CONSTRUCT, INSERT, DELETE, ASK, DESCRIBE?

9 SPARQL - Query Syntax PREFIX: the namespace prefixes used in the SPARQL query PREFIX dbo: < SELECT?city WHERE {?city dbo:areacode "020". } LIMIT 10 SELECT: the entities (variables) you want to return WHERE: the (sub)graph you want to get information from... including additional constraints on results (using operators)

10 SPARQL - Graph Pattern The WHERE clause specifies a graph pattern that should be matched can match multiple times A graph pattern is an RDF graph with some nodes & edges as variables dbo:country "020"^^xsd:string rdf:type??? dbo:capital

11 SPARQL - Triple Patterns A graph pattern consists of multiple triple patterns A triple pattern is a triple with zero or more variables?x dbo:capital dbpedia:amsterdam.?x dbo:capital?y.?x dbo:areacode "020".?x?p?y dbo:country "020"^^xsd:string dbpedia:netherlands rdf:type dbo:country ; dbo:capital dbpedia:amsterdam. rdf:type dbo:areacode dbpedia:amsterdam dbo:areacode "020". dbpedia:the_netherlands dbo:capital dbpedia:amsterdam

12 SPARQL - Triple patterns form a conjunction Every triple pattern should match PREFIX dbo: < SELECT?x WHERE {?x dbo:capital?y.?y dbo:areacode "020". } LIMIT 10 dbo:country "020"^^xsd:string rdf:type? dbo:capital?? dbo:country rdf:type dbpedia:the_netherlands "020"^^xsd:string dbo:areacode dbo:capital dbpedia:amsterdam "020"^^xsd:string dbo:areacode dbpedia:westminster

13 SPARQL - But we can also use disjunctions At least one of the graph patterns should match PREFIX dbo: < SELECT?y WHERE { {?y dbo:areacode "020". } UNION {?y dbo:areacode "010". } } LIMIT 10 "020"^^xsd:string "010"^^xsd:string dbo:areacode dbo:areacode dbpedia:amsterdam dbpedia:rotterdam

14 SPARQL - Optional graphs A part of the graph pattern is optional, and does not need to match PREFIX dbo: < SELECT?x?y WHERE {?y dbo:areacode "020". OPTIONAL {?x dbo:capital?y. } } LIMIT 10 dbo:country "020"^^xsd:string "020"^^xsd:string rdf:type dbo:areacode & dbo:areacode dbpedia:the_netherlands dbo:capital dbpedia:amsterdam dbpedia:westminster

15 Filtering query results Tests in the FILTER clause have to be validated for matching subgraphs PREFIX dbo: < SELECT?y WHERE {?y dbo:areacode "020".?y dbo:populationtotal?x. FILTER (?x > ) } "020"^^xsd:string dbo:areacode dbpedia:amsterdam dbo:populationtotal "790654"^^xsd:integer "020"^^xsd:string dbo:areacode dbpedia:borehamwood dbo:populationtotal "28546"^^xsd:integer

16 SPARQL - Solution modifiers Sorting using ORDER BY PREFIX dbo: < PREFIX rdfs: < SELECT?label WHERE {?place dbo:areacode "020"; rdfs:label?label. } ORDER BY DESC (?label) LIMITing the number of results PREFIX dbo: < PREFIX rdfs: < SELECT?label WHERE {?y dbo:areacode "020"; rdfs:label?label. FILTER (lang(?label) = "en" ) } ORDER BY DESC(?label) LIMIT 10 DISTINCT results PREFIX dbo: < PREFIX dbpedia: < PREFIX rdf: < SELECT DISTINCT?name WHERE {?country rdf:type dbo:country ; dbo:capital?ams.?ams dbo:country dbpedia:netherlands ; rdfs:label?name. FILTER (lang(?name) = "en" ) }

17 SPARQL - Property Paths Not interested in variable bindings Transitively walk over the same property relation Are interested in alternatives without having to write a UNION PREFIX dbo: < PREFIX dbpedia: < PREFIX rdf: < PREFIX skos: < SELECT DISTINCT?name WHERE {?country rdf:type dbo:country ; dbo:capital PREFIX dbo:?city < {?city rdfs:label PREFIX dbpedia:?name < } UNION PREFIX rdf: < {?city skos:preflabel PREFIX skos:?name < } FILTER (lang(?name) = "en" ) } SELECT DISTINCT?name WHERE {?country rdf:type dbo:country ; dbo:capital/(rdfs:label skos:preflabel)?name. } FILTER (lang(?name) = "en" ) }

18 SELECT returns a table with variable bindings CONSTRUCT returns a RDF graph PREFIX dbo: < PREFIX ex: < CONSTRUCT {?y ex:kengetal "020". } WHERE {?y dbo:areacode "020". } ASK returns true or false DESCRIBE returns a RDF graph INSERT is like CONSTRUCT, but inserts the graph into the triple store PREFIX dbo: < PREFIX dbr: < ASK WHERE { dbr:amsterdam dbo:areacode "020". } PREFIX dbr: < DESCRIBE dbr:amsterdam PREFIX dbo: < PREFIX ex: < INSERT {?y ex:kengetal "020". } WHERE {?y dbo:areacode "020". } SPARQL Update

19 SPARQL - More features Named Graphs provide context for statements they contain GRAPH < { } Aggregates COUNT, SUM, MIN, MAX, AVG, GROUP_CONCAT and SAMPLE Grouping and group aggregates GROUP BY and HAVING Negation MINUS Functions IF, BOUND, EXISTS, NOT EXISTS, IN, NOT IN &&,, =, <, >, =>, =< isiri, isblank, isliteral, isnumeric, str, lang, datatype STRLEN, SUBSTR, CONTAINS, CONCAT, REGEX now, year, month, day, hours, minutes, seconds and more Provide data inline VALUES

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21 SPARQL - Querying the Linked Data Cloud DBPedia + = LOD Cache: DBPedia + Geonames + NY Times + MusicBrainz +... Same query, different results! PREFIX dbo: < PREFIX owl: < SELECT DISTINCT?x?y WHERE {?x dbo:areacode "020".?y owl:sameas?x. FILTER (?x!=?y) }

22 SPARQL - Summary Linked Data is usually stored in triple stores SPARQL is the query language for the Web of Linked Data Queries are sent to SPARQL endpoints over HTTP Queries describe graph patterns with variables Graph patterns match (parts of) the RDF graphs stored in the triple store Results are returned as a table with variable bindings (JSON, XML, TSV, CSV )

23 Linked Data Lifecycle Interlinking Authoring Enrichment Storage/ Querying Linked (Open) Data Lifecycle Quality Analysis Extraction Evolution Exploration

24 Exploration

25 Exploration RDF - Vocabularies The RDF vocabulary and RDFS reserved terms needed for the data model Geonames locations Friend of a Friend (FOAF) persons and relations between persons Dublin Core (DC and DCTerms) bibliographic attributes (author, title, etc.) DBPedia Ontology DBPedia is at the heart of the Web of Data Data Cube (QB) statistical data Simple Knowledge Organization System (SKOS) concept hierarchies and mappings Web Ontology Language (OWL) formal constraints on class membership

26 Exploration RDF - The RDF Vocabulary Namespace < Usual prefix rdf (NB: needs to be declared, like all others) Important elements rdf:type links a resource to a type rdf:resource is the type of all resources rdf:property is the type of all predicates Examples geo:amsterdam rdf:type rdf:resource. geo:containedin a rdf:property. rdf:type a rdf:property.

27 Exploration RDFS - The RDF Schema Vocabulary Namespace < Usual prefix rdfs (NB: needs to be declared, like all others) Important elements rdfs:label links a resource to its name rdfs:class is the type of all types rdfs:subclassof inheritance over types rdfs:subpropertyof inheritance over properties Examples geo:amsterdam rdfs:label geo:containedin rdfs:subpropertyof ex:partof. geo:amsterdam a ex:city. ex:city rdfs:subclassof ex:place.

28 Exploration FOAF - The Friend-of-a-Friend Vocabulary Namespace < Usual prefix foaf Important elements foaf:person a person, alive, dead, imaginary foaf:name the name of a person foaf:mbox the URI of a person foaf:knows one person knows another person Examples vu:rinke_hoekstra rdf:type foaf:person ; foaf:knows vu:albert_merono ; foaf:name "Rinke Hoekstra".

29 Exploration DC - The Dublin Core Vocabulary Namespace < Usual prefix dct Important elements dct:creator a document's main author dct:created the creation date dct:description a natural language description dct:title the title of the document Examples vu:swprimer dct:creator vu:rinke_hoekstra ; dct:created "2012"^^xsd:date. dct:title "A Semantic Web ".

30 Exploration RDF Data Cube Vocabulary Published by the Government Linked Data Working Group Designed for multidimensional statistical data Inspired by existing standards (SDMX) and methods (OLAP) slice dimension ID surname age occupation sex married_to 1 Fumes 20 cigar maker female 25 observation 2 Bridges 32 civil engineer female 64 3 Moves 17 dancer male 325 code

31 Exploration RDF Data Cube Vocabulary Used to represent the data structure of datasets Identify the role of attributes (dimension, measure, annotation)... Predictable graph structure Bridges surname occupation... surname married_to 64 age age 2 married_to occupation sex female... civil engineer

32 RDF Data Cube Vocabulary Exploration

33 Interlinking A moment of reflection Can we use SPARQL for interlinking?

34 Hands On