RDF for Developers

1. Paul Groth RDF for Developers Thanks to Eyal Oren, Stefan Schlobach for slides

2. Contents • The Web of Data • A Data Model • A Query Language • Query Time… • Building Apps • Exposing Data

3. Why can’t we query all the information on the Web as a database?

4. Data Interoperability on the Web • Different formats, different structures, different vocabularies, different concepts, different meaning • Data should be structured (not ASCII) • Structure should be data-oriented (not HTML) • Meaning of data should be clear (not XML) • Data should have standard APIs (not Flickr) • Reusable mappings between data are needed (not XSLT)

5. What is the Semantic Web? Tim Berners-Lee “There is lots of data we all use every day, and it’s not part of the web. I can see my bank statements on the web, and my photographs, and I can see my appointments in a calendar. But can I see my photos in a calendar to see what I was doing when I took them? Can I see bank statement lines in a calendar? No. Why not? Because we don’t have a web of data. Because data is controlled by applications and each application keeps it to itself.”

6. The Web of Data

7. Enrich your data

8. Standard Data Model • RDF: basic data format (triples form hypergraph) • john likes mary . • isbn:10993 dc:title "The Pelican Brief" . • RDFS : simple schema language (subclass, subproperty) • dc:titlerdfs:subPropertyOfrdfs:label . • Jeep isa Car . • OWL: rich schema language (constraints, relations) • likes isaowl:symmetricProperty . • RDF allows interlinked graphs of arbitrary structure • RDFS and OWL allow inferencing of implicit information

9. Standard API: SPARQL • Query language for RDF • Based on existing ideas (e.g. SQL) • Supported widely • Standard query protocol • How to send a query over HTTP • How to respond over HTTP

10. RDF Basics

22. RDF Data Model: Summary • Resources • Triples • Statements: typed links between triples • May involve literals as property values • Graph • Set of Statements • Interlinked by reusing same URIsacrossed triples

23. RDF Syntax • Several Different Formats • Standard RDF/XML • NTriples • Turtle

25. SPARQL

26. SQL? • Formulate a query on the relational model • students(name, age, address) • Structured Query Language (SQL) SELECT namedata needed FROM studentdata source WHERE age>20data constraint

27. SPARQL • Standard RDF query language • based on existing ideas • standardised by W3C • widely supported • Standard RDF query protocol • how to send a query over HTTP • how to respond over HTTP

28. SPARQL Query Syntax • SPARQL uses a select-from-where inspired syntax (like SQL): • select: the entities (variables) you want to return SELECT ?city • from: the data source (RDF dataset) FROM <http://example.org/geo.rdf> • where: the (sub)graph you want to get the information from WHERE {?city geo:areacode “010” .} • Including additional constraints on objects, using operators WHERE {?city geo:areacode ?c. FILTER (?c > 010)} • prologue: namespace information PREFIX geo: <http://example.org/geo.rdf#>

29. SPARQL Query Syntax PREFIX geo: <http://example.org/geo/> SELECT ?city FROM <http://example.org/geoData.rdf> WHERE {?citygeo:areacode ?c . FILTER (?c > 010) }

30. EuropeanCountry “020”^^xsd:integer hasCapital type ? ? ? SPARQL Graph Patterns • The core of SPARQL • WHERE clause specifies graph pattern • pattern should be matched • pattern can match more than once • Graph pattern: • an RDF graph • with some nodes/edges as variables

31. Basis: triple patterns • Triples with one/more variables • Turtle syntax • ?X geo:hasCapitalgeo:Amsterdam • ?X geo:hasCapital ?Y • ?X geo:areacode "020" • ?X ?P ?Y • All of them match this graph: areacode hasCapital Netherlands Amsterdam “020”

32. Basis: triple pattern • A very basic query PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geoData.rdf> WHERE { ?X geo:hasCapital ?Y .}

33. Conjunctions: several patterns • A pattern with several graphs, all must match PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geoData.rdf> WHERE { {?X geo:hasCapital ?Y } {?Y geo:areacode "020" } } equivalent to PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geoData.rdf> WHERE { ?X geo:hasCapital ?Y . ?Y geo:areacode "020" . }

34. Conjunctions: several patterns • A pattern with several graphs, all must match PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geoData.rdf> WHERE { {?X geo:hasCapital ?Y } {?Y geo:areacode "020" } } equivalent to PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geoData.rdf> WHERE { ?X geo:hasCapital [ geo:areacode "020" ]. }

35. Note: Turtle syntax again • ?X geo:name ?Y ; geo:areacode ?Z . • ?X geo:name ?Y . ?X geo:areacode ?Z . • ?country geo:capital [ geo:name “Amsterdam” ] .

36. Alternative Graphs: UNION • A pattern with several graphs, at least one should match PREFIX geo: <http://example.org/geo/> SELECT ?city WHERE { { ?citygeo:name "Parijs"@nl } UNION { ?citygeo:name "Paris"@fr .} }

37. Optional Graphs • RDF is semi-structured • Even when the schema says some object can have a particular property, it may not always be present in the data • Example: persons can have names and email addresses, but Frank is a person without a known email address name person001 “Antoine” email “aisaac@few.vu.nl” name person002 “Frank”

38. Optional Graphs (2) • “Give me all people with first names, and if known their email address” • An OPTIONAL graph expression is needed PREFIX : <http://example.org/my#> SELECT ?person ?name ?email WHERE { ?person :name ?name . OPTIONAL { ?person :email ?email } }

39. Testing values of nodes • Tests inFILTERclause have to be validated for matching subgraphs • RDF model-related operators • isLiteral(?aNode) • isURI(?aNode) • STR(?aResource) Interest of STR? SELECT ?X ?N WHERE { ?X ?P ?N . FILTER (STR(?P)="areacode") } • For resources with names only partly known • For literals with unknown language tags

40. Testing values of nodes • Tests inFILTERclause • Comparison: • ?X <= ?Y, ?Z < 20, ?Z = ?Y, etc. • Arithmetic operators • ?X + ?Y, etc. • String matchingusing regular expressions • REGEX (?X,"netherlands","i") • matches"The Netherlands" PREFIX geo: <http://example.org/geo/> SELECT ?X ?N WHERE { ?X geo:name ?N . FILTER REGEX(STR(?N),"dam") }

41. Filtering results • Tests in FILTER clause • Boolean combination of these test expressions • && (and), || (or), ! (not) • (?Y > 10 && ?Y < 30) || !REGEX(?Z,"Rott") PREFIX geo: <http://example.org/geo/> SELECT ?X FROM <http://example.org/geo.rdf> WHERE {?X geo:areacode ?Y ; geo:name ?Z . FILTER ((?Y > 10 && ?Y < 30) || !REGEX(STR(?X),"Rott")) }

42. Boolean comparisons and datatypes • Reminder: RDF has basic datatypes for literals • XML Schema datatypes:xsd:integer, xsd:float, xsd:string, etc. • Datatypes can be used in value comparison • X < “21”^^xsd:integer • and be obtained from literals • DATATYPE(?aLiteral)

43. Solution modifiers • ORDER BY SELECT ?dog ?ageWHERE { ?dog a Dog ; ?dog :age ?age . } ORDER BY DESC(?age) • LIMITSELECT ?dog ?ageWHERE { ?dog a Dog ; ?dog :age ?age . }ORDER BY ?dogLIMIT 10

44. SELECT Query Results • SPARQL SELECT queries return solutions that consist of variable bindings • For each variable in the query, it gives a value (or a list of values). • The result is a table, where each column represents a variable and each row a combination of variable bindings

45. Query result: example • Query: “return all countries with the citiesthey contain, and their areacodes, if known” • Result (as table of bindings): PREFIX geo: <http://example.org/geo/> SELECT ?X ?Y ?Z WHERE { ?X geo:containsCity ?Y. OPTIONAL {?Y geo:areacode ?Z} }

46. SELECT Query results: format • Query: return all capital cities • Results as an XML document : PREFIX geo: <http://example.org/geo/> SELECT ?X ?Y WHERE { ?X geo:name ?Y .} <sparqlxmlns="http://www.w3.org/2005/sparql-results#"> <head> <variablename="X"/> <variablename="Y"/> </head> <results> <result> <bindingname="X"><uri>http://example.org/Paris</uri></binding> <bindingname="Y"><literal>Paris</literal></binding> </result> <result> <bindingname="X"><uri>http://example.org/Paris</uri></binding> <bindingname="Y"><literalxml:lang="nl">Parijs</literal></binding> </result> ... </results> </sparql> Header Results

47. Result formats <sparqlxmlns="http://www.w3.org/2005/sparql-results#"> <head> <variable name="x"/> <variable name="hpage"/> </head> <results> <result> <binding name=”x"> <literal datatype=”…/XMLSchema#integer">30</literal> </binding> <binding name="hpage"> <uri>http://work.example.org/bob/</uri> </binding> </result> </results> </sparql> • RDF/XML • N3 • JSON • TEXT

48. Query Result forms • SELECT queries return variable bindings • Do we need something else? • Statements from RDF original graph • Data extraction • New statements derived from original data according to a specific need • Data conversion, views over data

49. SPARQL CONSTRUCT queries • Construct-queries return RDF statements • The query result is either a subgraph of the original graph, or a transformed graph Subgraph query: PREFIX geo: <http://example.org/geo/> CONSTRUCT {?X geo:hasCapital ?Y } WHERE { ?X geo:hasCapital ?Y . ?Y geo:name "Amsterdam" } hasCapital Netherlands Amsterdam

50. SPARQL CONSTRUCT queries • Construct-queries return RDF statements • The query result is either a subgraph of the original graph, or a transformed graph Transformation query: PREFIX geo: <http://example.org/geo/> PREFIX my: <http://example.org/myNS/> CONSTRUCT {?Y my:inCountry ?X} WHERE { ?X geo:hasCapital ?Y} inCountry Netherlands Amsterdam