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 } . }

accesing a graph conditional on other information contained in the metadata about named graphs in the dataset

    PREFIX dc: <http://purl.org/dc/elements/1.1/> 
    PREFIX app: <http://example.org/ns#> 
    CONSTRUCT { ?s ?p ?o } 
    WHERE { GRAPH ?g { ?s ?p ?o } . 
             { ?g dc:publisher <http://www.w3.org/> } . 
             { ?g dc:date ?date } . 
    FILTER app:myDate(?date) > "2005-02-8T00:00:00Z"^^xsd:dateTime.                   }

    PREFIX ent: <http://myorg.example/employees#> DESCRIBE ?x
    WHERE { ?x ent:employeeId "1234" } 
    
    @prefix foaf: <http://xmlns.com/foaf/0.1/> . 
    @prefix vcard: <http://www.w3.org/2001/vcard-rdf/3.0> . 
    @prefix myOrg: <http://myorg.example/employees#> .
    _:a myOrg:employeeId "1234" ; 
    foaf:mbox_sha1sum "ABCD1234" ; 
    vcard:N [ vcard:Family "Smith" ; 
    vcard:Given "John" ] . 
    foaf:mbox_sha1sum rdf:type owl:InverseFunctionalProperty . 

    @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:homepage <http://work.example.org/alice/> . 
    _:b foaf:name "Bob" . 
    _:b foaf:mbox <mailto:bob@work.example> .

    PREFIX foaf: <http://xmlns.com/foaf/0.1/> 
    ASK { ?x foaf:name "Alice" } . 

• Named functions and syntactically constructed operations:
  • operands: subset of XML Schema DataTypes {xsd:string, xsd:decimal, xsd:double, xsd:dateTime} and types derived from xsd:decimal.
• Subset of XPath functions and operators
  • Operands: xs:string, xs:double, xs:float, xs:decimal, xs:integer, xs:dateTime
  • additional operators: sop:RDFterm-equal, sop:bound , sop:isURI, sop:isBlank, sop:isLiteral, sop:str , sop:lang, sop:datatype, sop:logical-or, sop:logical-and
• Type Promotion : xs:double, xs:float, xs:decimal
  • each of the numeric types is promoted to any type higher in the above list when used as an argument to function expecting that higher type.

Twinkle: A SPARQL Query Tool

Twinkle is a simple GUI interface that wraps the ARQ SPARQL query engine. The tool should be useful both for people wanting to learn the SPARQL query language, as well as those doing Semantic Web development.
Twinkle was originally inspired by Elliotte Harold's XQuisitor which provides a simple GUI interface for playing with XQuery.
Features:

• Load, edit and save SPARQL queries
• Insert PREFIX statements into queries
• Configure custom namespaces so they can be quickly inserted into queries
• Cancel long running queries
• Save results to file
• Query local files and remote RDF documents
• Query RDF data held in relational databases
• Query online SPARQL endpoints, such as DBpedia, reyvu.com and GovTrack.
• Query using standard SPARQL, or the ARQ extended syntax which supports COUNT, etc.
• Use ARQ extension functions and property functions
• Apply inferencing (e.g. Jena rules, RDF Schema, OWL ontology) when running queries
• Configure commonly used data sources for quick access

Redland RDF Libraries

Redland is a set of free software C libraries that provide support for the Resource Description Framework (RDF).

• Modular, object based libraries and APIs for manipulating the RDF graph, triples, URIs and Literals.
• Storage for graphs in memory and persistently with Sleepycat/Berkeley DB, MySQL 3-5, PostgreSQL, AKT Triplestore, SQLite, files or URIs.
• Support for multiple syntaxes for reading and writing RDF as RDF/XML, N-Triples and Turtle Terse RDF Triple Language, RSS and Atom syntaxes via the Raptor RDF Parser Library.
• Querying with SPARQL and RDQL using the Rasqal RDF Query Library.
• Data aggregation and recording provenance support with Redland contexts.
• Language Bindings in Perl, PHP, Python and Ruby via the Redland Bindings package.
• Command line utility programs rdfproc (RDF), rapper (parsing) and roqet (query).
• Portable, fast and with no known memory leaks.

The Redland library packages are:

• Raptor RDF Parser Library - libraptor for parsing and serializing RDF syntaxes

1.4.20 release raptor-1.4.20.tar.gz (2009-11-28)

• Rasqal RDF Query Library - librasqal for executing RDF queries

0.9.17 release rasqal-0.9.17.tar.gz (2009-12-16)

• Redland RDF Library - librdf providing the RDF API and triple stores. Requires: Raptor and Rasqal

1.0.10 release redland-1.0.10.tar.gz (2009-12-16)

• Redland Language Bindings for language APIs to Redland in Perl, PHP, Python and Ruby. Requires: Redland.

1.0.10.1 release redland-bindings-1.0.10.1.tar.gz (2009-12-16)

These are mature RDF packages developed since 2000 used in several projects. Each library has its own documentation and status information in the form of a list of TODOs/bugs, general news, detailed release notes and file-by-file changes in the ChangeLog.
License
All Redland packages are free software / open source software and released under the LGPL 2.1, GPL 2 or Apache 2 licenses as alternatives. See the individual package license files for full details and any exceptions.

ARQ

ARQ is a query engine for Jena that supports the SPARQL RDF Query language. SPARQL is the query language developed by the W3C RDF Data Access Working Group. ARQ Features:

• Standard SPARQL
• Free text search via Lucene
• SPARQL/Update
• Access and extension of the SPARQL algebra
• Support for custom filter functions
• Property functions for custom processing of semantic relationships
• Aggregation, GROUP BY and assignment as SPARQL extensions
• Support for federated query
• Support for extension to other storage systems
• Client-support for remote access to any SPARQL endpoint

Jena storage engines that plug into ARQ include:

• TDB – Custom storage engine for high performance applications
• SDB – Rewriter to SQL for transactional SPARQL on standard relational databases

Installation and Administration

Getting started

  The file parms.pl contains a number of settings relevant to the server. Notable the port to connect to, where to store user information, etc. Persistent data kept by the server is a list of users and their access rights (default users.db) and a file-based backup of the in-memory store (default in the directory SeRQL-store). Please check the content of parms.pl and follow directions in the comments. On Unix-like systems, edit run.pl to adjust the location of SWI-Prolog on the !# line. Next, start run.pl and launch the server using the command below.

    ?- serql_server.

Now direct your browser to the server, using the default setup this is http://localhost:3020. If no users are defined the browser will prompt to enter the administrative password. After that the admin and anonymous users are created. Accounts can be created and modified by users with administrative rights through the List users … link on the sidebar.
To restart from scratch, stop the server, delete the users database file and/or the triple backup file and restart the server as described above.

Persistent store The parms.pl setting persistent_store(Directory, Options) can be used to specify file-based persistent backup for the in-memory triple store. The store is a combination of quick-load triple databases and journal files that hold the modifications made to the triple store. Details of the persistent store are documented with the SWI-Prolog Semantic Web package

Roadmap
Query processing and entailment
The kernel of the system is formed by serql.pl and sparql, that implement the DCG parsers for the respective query languages as well as a compiler that translates this into a Prolog goal executing the query op top of the SWI-Prolog SemWeb package. The file rdfql_runtime.pl contains predicates that implement the constraints (SeRQL WHERE or SPARQL FILTER) and other constructs generated by the query-compiler.

Entailment reasoning is defined by rdf_entailment.pl. Specific entailments are in seperate files:

    no_entailment.pl

Defines entailment none. Query explicitely stored triples only.

    rdf_entailment.pl

Defines entailment rdf. Any resource appearing in a predicate position is of type rdf:Property. Any subject is an instance of rdf:Resource

    rdfs_entailment.pl

Defines entailment rdfs. Adds class- and property-hierarchy reasoning to RDF reasoning, as well as reasoning on the basis of property domain and range.

    rdfslite_entailment.pl

Defines entailment rdfslite. Only considers the class- and property-hierarchy. Using a backward chaining solver this is much faster, while normally keeping the intended meaning.

The query compiler and execution system can be called directly from Prolog.

    serql_compile(+Query, -Compiled, +Options)

Compile Query, which is either an atom or a list of character codes and unify Compiled with an opaque term representing the query and suitable for passing to serql_run/2. Defined Options are:

  entailment(Entailment)
      Entailment to use. Default is rdfs. See section 4.1.
  type(-Type)
      Extract the type of query compiled and generally useful information on it. SeRQL defines the types construct and select(VarNames), where VarNames is a list of variables appearing in the projection.
  optimise(Bool)
      Whether or not to optimise the query. Default is defined by the setting optimise_query. 

sparql_compile(+Query, -Compiled, +Options)

  Similar to to serql_compile/3. Defined types are extended with describe and ask. Addional options are:

  base_uri(-URI)
      Base URI used to compile the query if not specified as part of the query.
  ordered(-Bool)
      Unify Bool with true if query contains an ORDER BY clause.
  distinct(-Bool)
      Unify Bool with true if query contains a DISTINCT modifier. 

serql_run(+Compiled, -Answer)

  Run a query compiled by serql_compile/3, returning terms row(Arg ...) for select queries and terms rdf(Subject, Predicate, Object) for construct queries. Subsequent results are returned on backtracking.

sparql_run(+Compiled, -Answer)

  Similar to serql_run/2. Queries of type describe return rdf-terms like construct. Queries of type ask return either true or false.

serql_query(+Query, -Answer, +Options)

  Utility combining of serql_compile/3 and serql_run/2. Note this gives no access to the column-names.

sparql_query(+Query, -Answer, +Options)

  Similar to serql_query/3.