RDF y SPARQL: Dos componentes básicos para la Web de datos
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- Ashley Simmons
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1 RDF y SPARQL: Dos componentes básicos para la Web de datos Marcelo Arenas PUC Chile & University of Oxford M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
2 Semantic Web The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation. [Tim Berners-Lee et al ] Specific goals: Build a description language with standard semantics Make semantics machine-processable and understandable Incorporate logical infrastructure to reason about resources W3C proposals: Resource Description Framework (RDF) and SPARQL M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
![] Specific goals: Build a description language with standard semantics Make semantics machine-processable and understandable Incorporate](/docs-images/45/2755798/images/page_2.jpg "logical infrastructure to reason about resources W3C proposals: Resource Description Framework (RDF) and SPARQL M.")
3 RDF in a nutshell RDF is the framework proposed by the W3C to represent information in the Web: URI vocabulary A URI is an atomic piece of data, and it identifies an abstract resource Syntax based on directed labeled graphs URIs are used as node labels and edge labels Schema definition language (RDFS): Define new vocabulary Typing, inheritance of classes and properties,... Formal semantics M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
4 An example of an RDF graph: DBLP : < conf: < inpods: < swrc: < dc: < dct: < conf:pods "Optimal Aggregation..." swrc:series inpods:2001 dct:partof dc:title inpods:faginln01 dc:creator dc:creator dc:creator :Amnon Lotem :Moni Naor :Ronald Fagin M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
5 An example of a URI M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
6 URI can be used for any abstract resource Fagin M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
7 Querying RDF Why is this an interesting problem? Why is it challenging? RDF graphs can be interconnected URIs should be dereferenceable Semantics of RDF is open world RDF graphs are inherently incomplete The possibility of adding optional information if present is an important feature Vocabulary with predefined semantics... M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
8 Querying RDF: SPARQL SPARQL is the W3C recommendation query language for RDF (January 2008). SPARQL is a recursive acronym that stands for SPARQL Protocol and RDF Query Language SPARQL is a graph-matching query language. A SPARQL query consists of three parts: Pattern matching: optional, union, filtering,... Solution modifiers: projection, distinct, order, limit, offset,... Output part: construction of new triples,... M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
9 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
10 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
11 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
12 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
13 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
14 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
15 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } A SPARQL query consists of a: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
16 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } A SPARQL query consists of a: Body: Pattern matching expression M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
17 SPARQL in a nutshell SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } A SPARQL query consists of a: Body: Pattern matching expression Head: Processing of the variables M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
18 What are the challenges in implementing SPARQL? SPARQL has to take into account the distinctive features of RDF: Should be able to extract information from interconnected RDF graphs Should be consistent with the open-world semantics of RDF Should offer the possibility of adding optional information if present Should be able to properly interpret RDF graphs with a vocabulary with predefined semantics M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
19 Extracting information from interconnected RDF graphs : < dbpedia: < rdf: < rdfs: < owl: < yago: < dbo: < inpods:faginln01 dc:creator DBLP DBpedia dbpedia:ronald Fagin rdf:type yago:databaseresearchers owl:sameas :Ronald Fagin dbo:birthplace dbpedia:oklahoma rdfs:subclassof yago:researchworker M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
20 Dereferenceable URIs are the glue Fagin M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
21 Querying interconnected RDF graphs Retrieve the authors that have published in PODS and were born in Oklahoma: SELECT?Author WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods.?person owl:sameas?author.?person dbo:birthplace dbpedia:oklahoma. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
22 Retrieving optional information Retrieve the authors that have published in PODS, and their Web pages if this information is available: SELECT?Author?WebPage WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. OPTIONAL {?Author foaf:homepage?webpage. } } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
23 Taking into account vocabularies with predefined semantics Retrieve the scientists that were born in Oklahoma and that have published in PODS: SELECT?Author WHERE {?Author rdf:type yago:scientist.?author dbo:birthplace dbpedia:oklahoma.?paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
24 Taking into account vocabularies with predefined semantics Retrieve the scientists that were born in Oklahoma and that have published in PODS: dbpedia:ronald Fagin rdf:type yago:databaseresearchers dbo:birthplace rdfs:subclassof dbpedia:oklahoma yago:researchworker rdfs:subclassof yago:scientist M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
25 Outline of the talk RDF SPARQL: Syntax and semantics RDFS: RDF Schema Some new features in SPARQL 1.1 Concluding remarks M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
26 Outline of the talk RDF SPARQL: Syntax and semantics RDFS: RDF Schema Some new features in SPARQL 1.1 Concluding remarks M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
27 RDF formal model U Subject Predicate Object U : set of URIs B : set of blank nodes U B U B L L : set of literals M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
28 RDF formal model U Subject Predicate Object U : set of URIs B : set of blank nodes U B U B L L : set of literals (s,p,o) (U B) U (U B L) is called an RDF triple M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
29 RDF formal model U Subject Predicate Object U : set of URIs B : set of blank nodes U B U B L L : set of literals (s,p,o) (U B) U (U B L) is called an RDF triple A set of RDF triples is called an RDF graph M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
30 RDF formal model Proviso In this talk, we do not consider blank nodes (s,p,o) U U (U L) is called an RDF triple M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
31 Outline of the talk RDF SPARQL: Syntax and semantics RDFS: RDF Schema Some new features in SPARQL 1.1 Concluding remarks M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
32 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { P1 P2 } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
33 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { { P1 P2 } } { P3 P4 } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
34 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { { P1 P2 OPTIONAL { P5 } } } { P3 P4 OPTIONAL { P7 } } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
35 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { { P1 P2 OPTIONAL { P5 } } } { P3 P4 OPTIONAL { P7 OPTIONAL { P8 } } } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
36 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { { P1 P2 OPTIONAL { P5 } } { P3 P4 OPTIONAL { P7 OPTIONAL { P8 } } } } UNION { P9 } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
37 SPARQL queries can be complex Interesting features: Grouping Optional parts Nesting Union of patterns Filtering { { P1 P2 OPTIONAL { P5 } } { P3 P4 OPTIONAL { P7 OPTIONAL { P8 } } } } UNION { P9 FILTER ( R ) } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
38 SPARQL: An algebraic syntax Graph pattern:?x name?y (?X, name,?y) { P1. P2 } (P 1 AND P 2 ) { P1 OPTIONAL { P2 }} (P 1 OPT P 2 ) { P1 } UNION { P2 } (P 1 UNION P 2 ) { P1 FILTER ( R ) } (P 1 FILTER R ) SPARQL query: SELECT?X?Y... { P } (SELECT {?X,?Y,...} P) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
39 SPARQL: An algebraic syntax (cont d) Explicit precedence/association Example { t1 t2 OPTIONAL { t3 } OPTIONAL { t4 } t5 } ((((t 1 AND t 2 ) OPT t 3 ) OPT t 4 ) AND t 5 ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
40 Mappings: building block for the semantics Definition A mapping is a partial function: µ : V (U L) The evaluation of a graph pattern results in a set of mappings. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
41 Mappings: building block for the semantics Definition A mapping is a partial function: µ : V (U L) The evaluation of a graph pattern results in a set of mappings. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
42 The semantics of triple patterns Given an RDF graph G and a triple pattern t. Definition The evaluation of t over G is the set of mappings µ such that: µ has as domain the variables in t: dom(µ) = var(t) µ makes t to match the graph: µ(t) G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
43 The semantics of triple patterns Given an RDF graph G and a triple pattern t. Definition The evaluation of t over G is the set of mappings µ such that: µ has as domain the variables in t: dom(µ) = var(t) µ makes t to match the graph: µ(t) G Example graph triple evaluation (R 1, name, john)?x?y (R 1, , [email protected]) (?X, name,?y) µ 1 : R 1 john (R 2, name, paul) µ 2 : R 2 paul M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
44 The semantics of triple patterns Given an RDF graph G and a triple pattern t. Definition The evaluation of t over G is the set of mappings µ such that: µ has as domain the variables in t: dom(µ) = var(t) µ makes t to match the graph: µ(t) G Example graph triple evaluation (R 1, name, john)?x?y (R 1, , [email protected]) (?X, name,?y) µ 1 : R 1 john (R 2, name, paul) µ 2 : R 2 paul M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
45 The semantics of triple patterns Given an RDF graph G and a triple pattern t. Definition The evaluation of t over G is the set of mappings µ such that: µ has as domain the variables in t: dom(µ) = var(t) µ makes t to match the graph: µ(t) G Example graph triple evaluation (R 1, name, john)?x?y (R 1, , [email protected]) (?X, name,?y) µ 1 : R 1 john (R 2, name, paul) µ 2 : R 2 paul M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
46 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
47 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
48 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 µ 1 µ 2 : R 1 john [email protected] M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
49 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 µ 1 µ 2 : R 1 john [email protected] M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
50 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 µ 1 µ 2 : R 1 john [email protected] µ 1 µ 3 : R 1 john [email protected] R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
51 Compatible mappings Definition Mappings µ 1 and µ 2 are compatible if they agree in their common variables: If?X dom(µ 1 ) dom(µ 2 ), then µ 1 (?X) = µ 2 (?X). Example?X?Y?Z?V µ 1 : R 1 john µ 2 : R 1 [email protected] µ 3 : [email protected] R 2 µ 1 µ 2 : R 1 john [email protected] µ 1 µ 3 : R 1 john [email protected] R 2 µ 2 and µ 3 are not compatible M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
52 Sets of mappings and operations Let Ω 1 and Ω 2 be sets of mappings. Definition Join: extends mappings in Ω 1 with compatible mappings in Ω 2 Ω 2 = {µ 1 µ 2 µ 1 Ω 1, µ 2 Ω 2 and µ 1, µ 2 are compatible} Ω 1 Difference: selects mappings in Ω 1 that cannot be extended with mappings in Ω 2 Ω 1 Ω 2 = {µ 1 Ω 1 there is no mapping in Ω 2 compatible with µ 1 } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
53 Sets of mappings and operations Definition Union: includes mappings in Ω 1 and in Ω 2 Ω 1 Ω 2 = {µ µ Ω 1 or µ Ω 2 } Left Outer Join: extends mappings in Ω 1 with compatible mappings in Ω 2 if possible Ω 1 Ω 2 = (Ω 1 Ω 2 ) (Ω 1 Ω 2 ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
54 Semantics of SPARQL Given an RDF graph G. Definition t G = (P 1 AND P 2 ) G = (P 1 UNION P 2 ) G = (P 1 OPT P 2 ) G = (SELECT W P) G = M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
55 Semantics of SPARQL Given an RDF graph G. Definition t G = {µ dom(µ) = var(t) and µ(t) G} (P 1 AND P 2 ) G = P 1 G P 2 G (P 1 UNION P 2 ) G = P 1 G P 2 G (P 1 OPT P 2 ) G = P 1 G P 2 G (SELECT W P) G = {µ W µ P G } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
56 Semantics of SPARQL Given an RDF graph G. Definition t G = {µ dom(µ) = var(t) and µ(t) G} (P 1 AND P 2 ) G = P 1 G P 2 G (P 1 UNION P 2 ) G = P 1 G P 2 G (P 1 OPT P 2 ) G = P 1 G P 2 G (SELECT W P) G = {µ W µ P G } dom(µ W ) = dom(µ) W and µ W (?X) = µ(?x) for every?x dom(µ W ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
57 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e)) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
58 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e)) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
59 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
60 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
61 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?E R 1 [email protected] M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
62 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?E R 1 [email protected] M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
63 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?Y?E R 1 john [email protected] paul R 2?X?E R 1 [email protected] M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
64 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?Y?E R 1 john [email protected] paul R 2?X?E R 1 [email protected] from the Join M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
65 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?Y?E R 1 john [email protected] paul R 2?X?E R 1 [email protected] from the Difference M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
66 Semantics of SPARQL: An example Example (R 1, name, john) (R 1, , (R 2, name, paul) ((?X, name,?y) OPT (?X, ,?e))?x?y R 1 john paul R 2?X?Y?E R 1 john [email protected] paul R 2?X?E R 1 [email protected] from the Union M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
67 Filter expressions (value constraints) Filter expression: P FILTER R P is a graph pattern R is a built-in condition We consider in R: equality = among variables and RDF terms unary predicate bound boolean combinations (,, ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
68 Filter expressions (value constraints) Example Some filter expressions: (?X = conf:pods) (?X = conf:pods) (?X = conf:pods) (?Y = conf:sigmod) (?X = conf:pods) (?Y = conf:sigmod) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
69 Satisfaction of value constraints A mapping µ satisfies a condition R (µ = R) if: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
70 Satisfaction of value constraints A mapping µ satisfies a condition R (µ = R) if: R is?x = c,?x dom(µ) and µ(?x) = c R is?x =?Y,?X,?Y dom(µ) and µ(?x) = µ(?y) R is bound(?x) and?x dom(µ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
71 Satisfaction of value constraints A mapping µ satisfies a condition R (µ = R) if: R is?x = c,?x dom(µ) and µ(?x) = c R is?x =?Y,?X,?Y dom(µ) and µ(?x) = µ(?y) R is bound(?x) and?x dom(µ) R is R 1 and µ = R 1 does not hold R is (R 1 R 2 ), and µ = R 1 or µ = R 2 R is (R 1 R 2 ), µ = R 1 and µ = R 2 M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
72 Satisfaction of value constraints A mapping µ satisfies a condition R (µ = R) if: R is?x = c,?x dom(µ) and µ(?x) = c R is?x =?Y,?X,?Y dom(µ) and µ(?x) = µ(?y) R is bound(?x) and?x dom(µ) R is R 1 and µ = R 1 does not hold R is (R 1 R 2 ), and µ = R 1 or µ = R 2 R is (R 1 R 2 ), µ = R 1 and µ = R 2 Definition FILTER : selects mappings that satisfy a condition (P FILTER R) G = {µ P G µ = R} M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
73 Outline of the talk RDF SPARQL: Syntax and semantics RDFS: RDF Schema Some new features in SPARQL 1.1 Concluding remarks M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
74 Syntax of RDFS RDFS extends RDF with a schema vocabulary: subpropertyof (rdf:sp), subclassof (rdf:sc), domain (rdf:dom), range (rdf:range), type (rdf:type). M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
75 Syntax of RDFS RDFS extends RDF with a schema vocabulary: subpropertyof (rdf:sp), subclassof (rdf:sc), domain (rdf:dom), range (rdf:range), type (rdf:type). How can one query RDFS data? Evaluating queries which involve this vocabulary is challenging M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
76 A simple SPARQL query: (Messi, rdf:type, person) person rdf:dom works in rdf:range company rdf:sc sportman rdf:sp rdf:sc rdf:sc soccer player rdf:dom plays in rdf:range soccer team rdf:type rdf:type Messi Barcelona lives in Spain M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
77 Semantics of RDFS Checking whether a triple t is in a graph G is the basic step when answering queries over RDF. For the case of RDFS, we need to check whether t is implied by G The notion of entailment in RDFS can be defined in terms of classical notions such as model, interpretation, etc. As for the case of first-order logic This notion can also be characterized by a set of inference rules. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
78 An inference system for RDFS R Inference rule: R R and R are sequences of RDF triples including symbols A, X,..., to be replaced by elements from (U L) σ(r) Instantiation of a rule: σ(r ) σ : {A,X,...} (U L) Application of a rule R R to an RDF graph G: Select an assignment σ : {A,X,...} (U L) if σ(r) G, then obtain G σ(r ) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
79 An inference system for RDFS (cont d) Sub-property : Subclass : Typing : (A, rdf:sp, B) (B, rdf:sp, C) (A, rdf:sp, C) (A, rdf:sp, B) (X, A, Y) (X, B, Y) (A, rdf:sc, B) (B, rdf:sc, C) (A, rdf:sc, C) (A, rdf:sc, B) (X, rdf:type, A) (X, rdf:type, B) (A, rdf:dom, B) (X, A, Y) (X, rdf:type, B) (A, rdf:range, B) (X, A, Y) (Y, rdf:type, B) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
80 Entailment in RDFS The previous system of inference rules characterize the notion of entailment in RDFS (without blank nodes). Thus, a triple t can be deduced from an RDF graph G (G = t) if there exists an RDF G such that: t G G can be obtained from G by successively applying the rules in the previous system. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
81 Entailment in RDFS: Closure of a graph Definition The closure of an RDFS graph G (cl(g)) is the graph obtained by adding to G all the triples that are implied by G. A basic property of the closure: G = t iff t cl(g) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
82 Querying RDFS data Basic step for answering queries over RDFS: Checking whether a triple t is in cl(g) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
83 Querying RDFS data Basic step for answering queries over RDFS: Checking whether a triple t is in cl(g) Definition The RDFS-evaluation of a graph pattern P over an RDFS graph G is defined as the evaluation of P over cl(g): P rdfs G = P cl(g) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
84 Example: (Messi, rdf:type, person) over the closure person rdf:dom works in rdf:range company rdf:sc rdf:sc sportman rdf:sp rdf:sc rdf:sc rdf:type rdf:type soccer player rdf:dom plays in rdf:range soccer team rdf:type rdf:type Messi Barcelona lives in Spain M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
85 Answering SPARQL queries over RDFS A simple approach for answering a SPARQL query P over an RDF graph G: Compute cl(g), and then evaluate P over cl(g) as for RDF M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
86 Answering SPARQL queries over RDFS A simple approach for answering a SPARQL query P over an RDF graph G: Compute cl(g), and then evaluate P over cl(g) as for RDF This approach has some drawbacks: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
87 Answering SPARQL queries over RDFS A simple approach for answering a SPARQL query P over an RDF graph G: Compute cl(g), and then evaluate P over cl(g) as for RDF This approach has some drawbacks: The size of the closure of G can be quadratic in the size of G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
88 Answering SPARQL queries over RDFS A simple approach for answering a SPARQL query P over an RDF graph G: Compute cl(g), and then evaluate P over cl(g) as for RDF This approach has some drawbacks: The size of the closure of G can be quadratic in the size of G Once the closure has been computed, all the queries are evaluated over a graph which can be much larger than the original graph M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
89 Answering SPARQL queries over RDFS A simple approach for answering a SPARQL query P over an RDF graph G: Compute cl(g), and then evaluate P over cl(g) as for RDF This approach has some drawbacks: The size of the closure of G can be quadratic in the size of G Once the closure has been computed, all the queries are evaluated over a graph which can be much larger than the original graph The approach is not goal-oriented When evaluating (a, rdf:sc, b), a goal-oriented approach should not compute cl(g): It should just verify whether there exists a path from a to b in G where every edge has label rdf:sc M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
90 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
91 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
92 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. A query P over an RDF graph G is answered by navigating G (cl(g) is not computed) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
93 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. A query P over an RDF graph G is answered by navigating G (cl(g) is not computed) This approach has some advantages: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
94 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. A query P over an RDF graph G is answered by navigating G (cl(g) is not computed) This approach has some advantages: It is goal-oriented M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
95 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. A query P over an RDF graph G is answered by navigating G (cl(g) is not computed) This approach has some advantages: It is goal-oriented It has been used to design query languages for XML (e.g., XPath and XQuery). The results for these languages can be used here M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
96 Extending SPARQL with navigational capabilities The example (a, rdf:sc, b) suggests that a query language with navigational capabilities could be appropriate for RDFS. Possible approach: Extend SPARQL with navigational capabilities. A query P over an RDF graph G is answered by navigating G (cl(g) is not computed) This approach has some advantages: It is goal-oriented It has been used to design query languages for XML (e.g., XPath and XQuery). The results for these languages can be used here Navigational operators allow to express natural queries that are not expressible in SPARQL over RDFS M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
97 Outline of the talk RDF SPARQL: Syntax and semantics RDFS: RDF Schema Some new features in SPARQL 1.1 Concluding remarks M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
98 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
99 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
100 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions Entailment regime for RDFS M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
101 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions Entailment regime for RDFS Navigational capabilities: Property paths M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
102 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions Entailment regime for RDFS Navigational capabilities: Property paths Aggregates M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
103 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions Entailment regime for RDFS Navigational capabilities: Property paths Aggregates An operator SERVICE to distribute the execution of a query (SERVICE?X P) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
104 SPARQL 1.1 A new version of SPARQL has just been released (March 2013): SPARQL 1.1 Some new features in SPARQL 1.1: Nesting of SELECT expressions Entailment regime for RDFS Navigational capabilities: Property paths Aggregates An operator SERVICE to distribute the execution of a query (SERVICE?X P) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
105 Aggregates in SPARQL 1.1 SELECT COUNT(DISTINCT?Author) WHERE {?Paper dc:creator?author.?paper dct:partof?conf.?conf swrc:series conf:pods. } This query can be executed in the DBLP SPARQL endpoint. M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
106 SPARQL provides limited navigational capabilities Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
107 SPARQL provides limited navigational capabilities Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George (SELECT?X ((?X, friendof,?y) AND (?Y, name, George))) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
108 SPARQL provides limited navigational capabilities Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George (SELECT?X ((?X, friendof,?y) AND (?Y, name, George))) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
109 SPARQL provides limited navigational capabilities Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George (SELECT?X ((?X, friendof,?y) AND (?Y, name, George))) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
110 A possible solution: Property paths Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
111 A possible solution: Property paths Paul name URI 1 friendof friendof URI 0 Peter name phone name URI 2 friendof URI 3 name John [email protected] George (SELECT?X ((?X, (friendof),?y) AND (?Y, name, George))) M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
112 Navigational capabilities in SPARQL 1.1: Property paths Syntax of property paths: where a U exp := a exp/exp exp exp exp M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
113 Evaluating property paths The evaluation of a property path over an RDF graph G is defined as follows: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
114 Evaluating property paths The evaluation of a property path over an RDF graph G is defined as follows: a G = {(x,y) (x,a,y) G} M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
115 Evaluating property paths The evaluation of a property path over an RDF graph G is defined as follows: a G = {(x,y) (x,a,y) G} exp 1 /exp 2 G = {(x,y) z (x,z) exp 1 G and (z,y) exp 2 G } M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
116 Evaluating property paths The evaluation of a property path over an RDF graph G is defined as follows: a G = {(x,y) (x,a,y) G} exp 1 /exp 2 G = {(x,y) z (x,z) exp 1 G and (z,y) exp 2 G } exp 1 exp 2 G = exp 1 G exp 2 G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
117 Evaluating property paths The evaluation of a property path over an RDF graph G is defined as follows: a G = {(x,y) (x,a,y) G} exp 1 /exp 2 G = {(x,y) z (x,z) exp 1 G and (z,y) exp 2 G } exp 1 exp 2 G = exp 1 G exp 2 G exp G = {(a,a) a is a URI in G} exp G exp/exp G exp/exp/exp G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
118 Property paths in SPARQL 1.1 New element in SPARQL 1.1: A triple of the form (x,exp,y) exp is a property path x (resp. y) is either an element from U or a variable M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
119 Property paths in SPARQL 1.1 New element in SPARQL 1.1: A triple of the form (x,exp,y) exp is a property path x (resp. y) is either an element from U or a variable Example (?X, (rdf:sc),?y): Verifies whether?x is a subclass of?y (?X, (rdf:sp),?y): Verifies whether?x is a subproperty of?y M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
120 Semantics of property paths Evaluation of t = (?X,exp,?Y) over an RDF graph G is the set of mappings µ such that: M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
121 Semantics of property paths Evaluation of t = (?X,exp,?Y) over an RDF graph G is the set of mappings µ such that: The domain of µ is {?X,?Y}, and (µ(?x),µ(?y)) exp G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
122 Semantics of property paths Evaluation of t = (?X,exp,?Y) over an RDF graph G is the set of mappings µ such that: The domain of µ is {?X,?Y}, and (µ(?x),µ(?y)) exp G Example (?X, KLM/(KLM),?Y): It is possible to go from?x to?y by using the airline KLM ((?X, KLM/(KLM),?Y) FILTER (?X =?Y)): Same as before, but now?x,?y must be different M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
123 SPARQL 1.1 and RDFS Property paths can help in encoding the semantics of RDFS. Given a SPARQL graph pattern P, we would like to find a SPARQL 1.1 graph pattern Q such that: For every RDF graph G: P rdfs G = Q G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
124 SPARQL 1.1 and RDFS Property paths can help in encoding the semantics of RDFS. Given a SPARQL graph pattern P, we would like to find a SPARQL 1.1 graph pattern Q such that: For every RDF graph G: P rdfs G = Q G We already saw how to encode rdf:sc and rdf:sp. We will consider the example P = (?X,a,?Y), where a U {rdf:sc,rdf:sp,rdf:type,rdf:dom,rdf:range} M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
125 The case of P = (?X,a,?Y) What are the difficulties in this case??y rdf:sp rdf:sp rdf:sp a?x M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
126 The case of P = (?X,a,?Y) (cont d) Let Q be: ( SELECT {?X,?Y} ) ((?X,?Z,?Y) AND (?Z,(rdf:sp),a)) Then for every RDF graph G: P rdfs G = Q G M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
127 Are we done? List the pairs a,b of cities such that there is a way to travel from a to b. transportation service rdf:sp rdf:sp rdf:sp bus service flight service train service rdf:sp rdf:sp rdf:sp the Airline British Airways Renfe Oxford London Madrid Valladolid M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
128 Concluding remarks We have witnessed a constant growth in the amount of RDF data available on the Web Two fundamental components of the Semantic Web: RDF and SPARQL Some of the distinctive features of RDF have made the study and implementation of SPARQL challenging SPARQL is still under development: SPARQL 1.1,... M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
129 Thank you! M. Arenas RDF y SPARQL: Dos componentes básicos para la Web de datos Valladolid / 61
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