Sławomir Widlarz swidlarz@gmail.pl

* research on SPARQL,
* provide a presentation,
* look for language support
* other tools
* look for/build Prolog API, e.g. SeRQL

Resource Description Framework (RDF) sets a standard for describing metadata in a way that computers can understand and process it, and integrate information from disparate information sources on the Web. RDF has needed a standard query language for some time and having one will make many development tasks much easier. Or how would you feel using relational databases without SQL?

SPARQL (‘sparkle’) is a standardized query language for RDF data. It offers developers of Semantic Web applications a way to write queries across this wide range of information. Used with a common protocol, applications can access and combine information from multiple sources on the Web. For example, SPARQL can be used to write Semantic Web applications in areas like financial markets, tourism, transportation, and pharmaceuticals.

SPARQL, which is both a query language and a data access protocol, has the ability to become a key component in Web 2.0 applications: as a standard backed by a flexible data model, it can provide a common query mechanism for all Web 2.0 applications – just what Web 2.0 needs. As SPARQL emerges, we can expect to see a fast growing number of application developers

• RDF – flexible and extensible way to represent information about WWW resources
• SPARQL - query language for getting information from RDF graphs. It provides facilities to:
  • extract information in the form of URIs, blank nodes, plain and typed literals.
  • extract RDF subgraphs.
  • construct new RDF graphs based on information in the queried graphs
• matching graph patterns
• variables – global scope; indicated by ‘?‘ or ‘$‘
• query terms – based on Turtle syntax
• terms delimited by „<>” are relative URI references
• data description format - Turtle

Basic Graph Pattern – set of Triple Patterns

Group Pattern - a set of graph patterns must all match

Value Constraints - restrict RDF terms in a solution

Optional Graph Patterns .- additional patterns may extend the solution

Alternative Graph Pattern – two or more possible patterns are tried

Patterns on Named Graphs - patterns are matched against named graphs

• Set of Triple Patterns
  • Triple Pattern – similar to an RDF Triple (subject, predicate, object), but any component can be a query variable; literal subjects are allowed,
  • Matching a triple pattern to a graph: bindings between variables and RDF Terms.
• Matching of Basic Graph Patterns
  • A Pattern Solution of Graph Pattern GP on graph G is any substitution S such that S(GP) is a subgraph of G.

   SELECT ?x ?v WHERE { ?x ?x ?v }
   rdf:type rdf:type rdf:Property

Data:

    @prefix foaf:  <http://xmlns.com/foaf/0.1/> .

    _:a  foaf:name   "Johnny Lee Outlaw" .
    _:a  foaf:mbox   <mailto:jlow@example.com> .
    _:b  foaf:name   "Peter Goodguy" .
    _:b  foaf:mbox   <mailto:peter@example.org> .
    _:c  foaf:mbox   <mailto:carol@example.org> .

Query:

    PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
    SELECT ?name ?mbox
    WHERE
    { ?x foaf:name ?name .
    ?x foaf:mbox ?mbox }

Query Result:

Data:

   @prefix foaf:  <http://xmlns.com/foaf/0.1/> .\\

   _:a  foaf:name   "Alice" .\\
   _:b  foaf:name   "Bob" .\\

Query:

   PREFIX foaf:   <http://xmlns.com/foaf/0.1/>\\
   SELECT ?x ?name\\
   WHERE  { ?x foaf:name ?name }\\

In a SPARQL query string, a group graph pattern is delimited with braces: {}. For example, this query's query pattern is a group graph pattern of one basic graph pattern.

    PREFIX foaf:    <http://xmlns.com/foaf/0.1/> \\
    SELECT ?name ?mbox\\
    WHERE  { 
    ?x foaf:name ?name . 
    ?x foaf:mbox ?mbox .
            } 

The same solutions would be obtained from a query that grouped the triple patterns into two basic graph patterns. For example, the query below has a different structure but would yield the same solutions as the previous query:

     PREFIX foaf:    <http://xmlns.com/foaf/0.1/> \\
     SELECT ?name ?mbox \\
     WHERE  { 
       { ?x foaf:name ?name . } 
       { ?x foaf:mbox ?mbox . } 
            } 

Constraints can be given in an optional graph pattern. For example:

    {
    @prefix dc:   <http://purl.org/dc/elements/1.1/> . 
    @prefix :     <http://example.org/book/>   
    @prefix ns:   <http://example.org/ns#> .
    }
    :book1  dc:title  "SPARQL Tutorial" .
    :book1  ns:price  42 .
    :book2  dc:title  "The Semantic Web" .
    :book2  ns:price  23 .

Query:

    PREFIX  dc:  <http://purl.org/dc/elements/1.1/>
    PREFIX  ns:  <http://example.org/ns#>
    SELECT  ?title ?price
    WHERE   { ?x dc:title ?title .
    OPTIONAL { ?x ns:price ?price . FILTER (?price < 30) }
    }

Query results:

    {
    @prefix dc:   <http://purl.org/dc/elements/1.1/> . 
    @prefix :     <http://example.org/book/>   
    @prefix ns:   <http://example.org/ns#> .
    }
    :book1  dc:title  "SPARQL Tutorial" .
    :book1  ns:price  42 .
    :book2  dc:title  "The Semantic Web" .
    :book2  ns:price  23 .

Query:

    PREFIX  dc:  <http://purl.org/dc/elements/1.1/>
    PREFIX  ns:  <http://example.org/ns#>
    SELECT  ?title ?price
    WHERE   { ?x dc:title ?title .
    OPTIONAL { ?x ns:price ?price . FILTER (?price < 30) }
    }

Query results:

Graph patterns are defined recursively. A graph pattern may have zero or more optional graph patterns, and any part of a query pattern may have an optional part. In this example, there are two optional graph patterns. Data:

    @prefix foaf:       <http://xmlns.com/foaf/0.1/> .
    
    _:a  foaf:name       "Alice" .
    _:a  foaf:homepage   <http://work.example.org/alice/> .
    
    _:b  foaf:name       "Bob" .
    _:b  foaf:mbox       <mailto:bob@work.example> .

Query:

    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name ?mbox ?hpage
    WHERE  { ?x foaf:name  ?name .
             OPTIONAL { ?x foaf:mbox ?mbox } .
             OPTIONAL { ?x foaf:homepage ?hpage }
           }

Query result:

SPARQL provides a means of combining graph patterns so that one of several alternative graph patterns may match. If more than one of the alternatives matches, all the possible pattern solutions are found.

Pattern alternatives are syntactically specified with the UNION keyword.

Data:

    @prefix dc10:  <http://purl.org/dc/elements/1.0/> .
    @prefix dc11:  <http://purl.org/dc/elements/1.1/> .
    
    _:a  dc10:title     "SPARQL Query Language Tutorial" .
    _:a  dc10:creator   "Alice" .
    
    _:b  dc11:title     "SPARQL Protocol Tutorial" .
    _:b  dc11:creator   "Bob" .
    
    _:c  dc10:title     "SPARQL" .
    _:c  dc11:title     "SPARQL (updated)" .

Query:

    PREFIX dc10:  <http://purl.org/dc/elements/1.0/>
    PREFIX dc11:  <http://purl.org/dc/elements/1.1/>

    SELECT ?title
    WHERE  { { ?book dc10:title  ?title } UNION { ?book dc11:title  ?title } }

Query result:

This will return results with the variable x bound for solutions from the left branch of the UNION, and y bound for the solutions from the right branch. If neither part of the UNION pattern matched, then the graph pattern would not match.

    PREFIX dc10:  <http://purl.org/dc/elements/1.0/>
    PREFIX dc11:  <http://purl.org/dc/elements/1.1/>
    
    SELECT ?title ?author
    WHERE  { { ?book dc10:title ?title .  ?book dc10:creator ?author }
             UNION
             { ?book dc11:title ?title .  ?book dc11:creator ?author }
           }

• RDF data stores may hold multiple RDF graphs:
  • record information about each graph
  • queries that involve information from more than one graph
  • RDF Dataset in SPARQL terminology
  • the background graph, which does not have a name, and zero or more named graphs, identified by URI reference

• the relationship between named and background graphs:
  • (i) to have information in the background graph that includes provenance information about the named graphs (the application is not directly trusting the information in the named graphs )
  • (ii) to include the information in the named graphs in the background graph as well.

    # Background graph 
    @prefix dc: <http://purl.org/dc/elements/1.1/> . 
    <http://example.org/bob> dc:publisher "Bob" . 
    <http://example.org/alice> dc:publisher "Alice" . 

    # Graph: http://example.org/bob 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:a foaf:name "Bob" . 
    _:a foaf:mbox <mailto:bob@oldcorp.example.org> .

    # Graph: http://example.org/alice 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:a foaf:name "Alice" . 
    _:a foaf:mbox <mailto:alice@work.example.org> .

    # Background graph 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:x foaf:name "Bob" . 
    _:x foaf:mbox <mailto:bob@oldcorp.example.org> .
    _:y foaf:name "Alice" . 
    _:y foaf:mbox <mailto:alice@work.example.org> . 

    # Graph: http://example.org/bob 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:a foaf:name "Bob" . 
    _:a foaf:mbox <mailto:bob@oldcorp.example.org> .

    # Graph: http://example.org/alice 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:a foaf:name "Alice" . 
    _:a foaf:mbox <mailto:alice@work.example.org> .

The use of GRAPH changes the active graph for matching basic graph patterns within part of the query. Outside the use of GRAPH, the default graph is matched by basic graph patterns.

    # Graph: http://example.org/foaf/aliceFoaf 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> .
    @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . 
    @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . 
    _:a foaf:name "Alice" . 
    _:a foaf:mbox <mailto:alice@work.example> . 
    _:a foaf:knows _:b . 
    _:b rdfs:seeAlso <http://example.org/foaf/bobFoaf> . 
    <http://example.org/foaf/bobFoaf> rdf:type foaf:PersonalProfileDocument . 
    _:b foaf:name "Bob" . 
    _:b foaf:mbox <mailto:bob@work.example> . 
    _:b foaf:age 32 . 

    # Graph: http://example.org/foaf/bobFoaf 
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . 
    @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
     _:1 foaf:mbox <mailto:bob@work.example> .
     _:1 rdfs:seeAlso <http://example.org/foaf/bobFoaf> . 
    _:1 foaf:age 35 . 
    <http://example.org/foaf/bobFoaf> rdf:type foaf:PersonalProfileDocument . 

Query:

    PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
    SELECT ?src ?bobAge 
    WHERE { GRAPH ?src 
          { 
            ?x foaf:mbox <mailto:bob@work.example> . 
            ?x foaf:age ?bobAge }
          }

Query results:

Query:

    PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
    PREFIX data: <http://example.org/foaf/> 
    SELECT ?age 
    WHERE 
    {
    GRAPH data:bobFoaf 
    {   
    ?x foaf:mbox <mailto:bob@work.example> . 
    ?x foaf:age ?age  } 
    }

Query results:

Query:

    PREFIX data: <http://example.org/foaf/> 
    PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
    PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
    PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> 
    SELECT ?mbox ?age ?ppd 
    WHERE 
    { GRAPH data:aliceFoaf 
    { ?alice foaf:mbox <mailto:alice@work.example> ;
      foaf:knows ?whom . 
      ?whom foaf:mbox ?mbox ; 
      rdfs:seeAlso ?ppd . 
      ?ppd a foaf:PersonalProfileDocument . } . 
      GRAPH ?ppd 
      { 
      ?w foaf:mbox ?mbox ; 
      foaf:age ?age 
      } 
    } 

Query results:

• Optional-1: an optional pattern that has a common variable with a(more) basic graph pattern(s) must be executed after the basic graph pattern(s)

• Optional-2: there can't be two optionals with a common variable, if that variable does not occur in a basic graph pattern as well

• Constraint: constraints are evaluated after variables are assigned values

• SELECT
  • returns all, or a subset of the variables bound in a query pattern match
  • formats : XML or RDF/XML
• CONSTRUCT
  • returns an RDF graph constructed by substituting variables in a set of triple templates
• DESCRIBE
  • returns an RDF graph that describes the resources found.
• ASK
  • returns whether a query pattern matches or not.

    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    _:a foaf:name "Alice" .
    _:a foaf:mbox <mailto:alice@example.org> .

    PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
    PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> 
    CONSTRUCT { <http://example.org/person#Alice> vcard:FN ?name } 
    WHERE { ?x foaf:name ?name } 

    @prefix vcard: <http://www.w3.org/2001/vcard-rdf/3.0#>.
    <http://example.org/person#Alice> vcard:FN "Alice" .

extracting a whole graph from the target RDF dataset

    CONSTRUCT { ?s ?p ?o } 
    WHERE { GRAPH <http://example.org/myGraph> { ?s ?p ?o } . }