Dr. Alexandra I. Cristea

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CS 253 Topics in Database Systems C4

• Dr. Alexandra I. Cristea
• http//www.dcs.warwick.ac.uk/acristea/

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• Previously we looked at
• XML, and its query language(s)
• RDF
• Next
• RDF query languages

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RDF query languages
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Proposals

• SPARQL
• http//www.w3.org/TR/rdf-sparql-query/
• RDQL
• http//www.w3.org/Submission/RDQL/
• RQL
• http//139.91.183.309090/RDF/RQL/
• SeRQL
• http//www.openrdf.org/doc/sesame/users/ch06.html
  
• Triple
• http//triple.semanticweb.org/
• N3
• http//www.w3.org/DesignIssues/Notation3
• Comparison of languages
• http//www.aifb.uni-karlsruhe.de/WBS/pha/rdf-query
  /rdfquery.pdf

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SeRQL
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Introduction SeRQL

• "Sesame RDF Query Language", pronounced "circle
• new RDF/RDFS query language
• currently being developed by Aduna as part of
  Sesame. http//www.openrdf.org/
• It combines (best?) features of other (query)
  languages (RQL, RDQL, N-Triples, N3) and adds
  some of its own.

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Sesame

• open source RDF framework with support for RDF
  Schema inferencing and querying.
• Originally, it was developed by Aduna (then known
  as Aidministrator) as a research prototype for
  the EU research project On-To-Knowledge.
• further developed and maintained by Aduna in
  cooperation with NLnet Foundation, developers
  from Ontotext, and a number of volunteer
  developers

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SeRQL's features

• Graph transformation.
• RDF Schema support.
• XML Schema datatype support.
• Expressive path expression syntax.
• Optional path matching.

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SeRQL basic building blocks

• RDF
• URIs,
• literals and
• variables URIs and literals
• variables

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Variables

• identified by names.
• must start with a letter or an underscore ('_')
• and can be followed by zero or more letters,
  numbers, underscores, dashes ('-') or dots ('.').
  
• Examples
• Var1
• _var2
• unwise.var-name_isnt-it
• SeRQL keywords are not allowed to be used as
  variable names.

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(reserved) Keywords

• Currently
• select, construct, from, where, using,
  namespace, true, false, not, and, or, like,
  label, lang, datatype, null, isresource,
  isliteral, sort, in, union, intersect, minus,
  exists, forall, distinct, limit, offset.
• case-insensitive, (unlike variable names).

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URIs

• full URIs
• abbreviated URIs (QNames)

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Full URIs

• must be surrounded with "lt" and "gt".
• Tend to be long (!!)
• Examples
• lthttp//www.openrdf.org/index.htmlgt
• lthttp//www.w3.org/1999/02/22-rdf-syntax-nstypegt
• ltmailtosesame_at_openrdf.orggt
• ltfile///C\rdffiles\test.rdfgt

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Abbreviated URIs (QNames)

• Components
• defined prefix (for the namespace) and a colon
  (),
• then the URI part that is not a namespace
• Examples
• sesameindex.html
• rdftype
• foafPerson

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Literals

• Parts
• label,
• language tag, and
• datatype
• Examples
• "foo"
• "foo"_at_en
• "ltfoo/gt"http//www.w3.org/1999/02/22-rdf-syntax-
  nsXMLLiteral
• "ltfoo/gt"rdfXMLLiteral

Optional, mutually exclusive
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Blank nodes

• RDF has a notion of blank nodes.
• nodes that are not labelled with a URI or
  literal.
• Interpretation (?) "there exists a node such
  that..."
• Blank nodes have internal identifiers
• Shortcut in SeRQL
• _bnode1
• Attention problem of non-portability!!!

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Path expressions

• expressions that match specific paths through an
  RDF graph
• usually, triples path expressions of length 1
• in SeRQL arbitrary length

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Basic path expressions

• Query
• persons who work for (companies that are) IT
  companies.

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Original (possible) RDF

• lt?xml version1.0?gt
• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-r
  df-syntax-ns" xmlnsfoo"http//www.mycompany.smt
  hg/company"gt
• ltrdfDescription abouthttp//www.mycompany.smthg
  /company/Persongt
• ltfooworksForgt rdfresourcehttp//www.mycompany.
  smthg/company/Company lt/fooworksForgt
• lt/rdfDescriptiongt
• ltrdfDescription abouthttp//www.mycompany.smthg
  /company/Companygt
• ltrdftypegtrdfresourcehttp//www.mycompany.smth
  g/company/CompanySchemaITCompanylt/rdftypegt
• lt/rdfDescriptiongt
• lt/rdfRDFgt

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Basic path expressions

• Query
• persons who work for (companies that are) IT
  companies.
• Person fooworksFor Company rdftype
  fooITCompany

ltfooworksForgt
ltrdftypegt
Company
Person
ltfooITCompanygt
Triple (length 1)
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Multiple Path Expressions

• Separated with commas
• Example
• Person exworksFor Company,
• Company rdftype exITCompany

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Non-interesting nodes

• Can be left empty
• Examples
• Person exworksFor rdftype exITCompany
• Painting expainted_by exname "Picasso"

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Path expression short cuts

• Multi-value nodes
• Branches
• Reified statements

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Multi-valued nodes

• Multiple objects
• subj1 pred1 obj1, obj2, obj3
• Multiple subjects
• subj1, subj2, subj3 pred1 obj1
• Condition disjoint !!

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Branches

• subj1 pred1 obj1
• pred2 obj2
• Equivalent to
• subj1 pred1 obj1,
• subj1 pred2 obj2

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Reified statements

• reifSubj reifPred reifObj pred obj
• Equivalent to
• _Statement rdftype rdfStatement,
  _Statement rdfsubject reifSubj, _Statement
  rdfpredicate reifPred, _Statement rdfobject
  reifObj, _Statement pred obj

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Optional Path Expressions

• Person exname Name
• exage Age
• exemail EmailAddress

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Queries in SeRQL

• Select queries
• returning a table of values, or a set of
  variable-value bindings.
• SELECT, FROM, WHERE, LIMIT, OFFSET and USING
  NAMESPACE
• Construct queries
• returns a true RDF graph
• CONSTRUCT, FROM, WHERE, LIMIT, OFFSET and USING
  NAMESPACE

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Select queries

• SELECT C FROM C rdftype rdfsClass
• returns all URIs of classes
• SELECT DISTINCT
• FROM Country1 exborders exborders
  Country2
• USING NAMESPACE ex lthttp//example.org/things
  gt

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Construct queries

• CONSTRUCT Parent exhasChild Child
• FROM Child exhasParent Parent
• USING NAMESPACE ex lthttp//example.org/things
  gt
• CONSTRUCT
• FROM SUB rdfssubClassOf SUPER
• This query extracts all rdfssubClassOf relations
  from an RDF graph.

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WHERE clause

• Optional
• Specifies Boolean constraints
• SELECT Country
• FROM Country expopulation Population
• WHERE Population lt "1000000"xsdpositiveInteger
  
• USING NAMESPACE ex lthttp//example.org/thingsgt

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Nested WHERE clauses

• Query 1 (normal WHERE-clause)
• SELECT Name, EmailAddress
• FROM Person foafname Name
• exemail
  EmailAddress
• WHERE EmailAddress LIKE "g"
• Query 2 (nested WHERE-clause)
• SELECT Name, EmailAddress
• FROM Person foafname Name
• exemail EmailAddress WHERE EmailAddress LIKE
  "g"
• at most one nested WHERE-clause per optional path
  expression, and at most one 'normal' WHERE-clause

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Results WHERE queries

• Query 1
• Name EmailAddress
• Giancarlo giancarlo_at_example.work
• Query 2 (nested WHERE)
• Name EmailAddress
• Michael 
• Rubens 
• Giancarlo "giancarlo_at_example.work"

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LIKE operator

• SELECT Country
• FROM Country exname Name
• WHERE Name LIKE netherlands" IGNORE CASE
• USING NAMESPACE ex lthttp//example.org/thingsgt

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Built-in predicates

• X serqldirectSubClassOf Y
• X serqldirectSubPropertyOf Y
• X serqldirectType Y

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Set combinatory operations

• Union
• Intersect
• Minus

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Union

• SELECT title
• FROM book dc10title title
• UNION
• SELECT title
• FROM book dc11title title
• USING NAMESPACE
• dc10 lthttp//purl.org/dc/elements/1.0/gt,
  dc11 lthttp//purl.org/dc/elements/1.1/gt

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Intersect

• SELECT creator
• FROM album dc10creator creator
• INTERSECT
• SELECT creator
• FROM album dc11creator creator
• USING NAMESPACE
• dc10 lthttp//purl.org/dc/elements/1.0/gt,
• dc11 lthttp//purl.org/dc/elements/1.1/gt

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Minus (difference)

• SELECT title
• FROM album dc10title title
• MINUS
• SELECT title
• FROM album dc10title title dc10creator
  creator
• WHERE creator like "Paul"
• USING NAMESPACE
• dc10 lthttp//purl.org/dc/elements/1.0/gt,
• dc11 lthttp//purl.org/dc/elements/1.1/gt

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NULL values

• SELECT
• FROM X Y Z
• WHERE isLiteral(Z) AND datatype(L) NULL
• to check that a literal doesn't have a datatype

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Query Nesting

• IN
• ANY, ALL
• EXISTS

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IN

• SELECT name
• FROM rdftype exPerson
• exname name
• WHERE name IN
• ( SELECT n
• FROM rdftype exAuthor
• exname n )
• USING NAMESPACE
• ex http//example.org/things
• retrieve all names of Persons, but only those
  names that also appear as names of Authors.

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ANY, ALL

• SELECT highestValue
• FROM node exvalue highestValue
• WHERE highestValue gt ALL
• ( SELECT value
• FROM exvalue value
• )
• USING NAMESPACE
• ex lthttp//example.org/thingsgt

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EXISTS

• SELECT name, hobby
• FROM rdftype exPerson
• exname name
• exhobby hobby
• WHERE EXISTS
• ( SELECT n
• FROM rdftype exAuthor
• exname n
• exauthorOf
• WHERE n name
• )
• USING NAMESPACE
• ex lthttp//example.org/thingsgt

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RDF Query Languages Conclusion

• We have learned
• There is a high competition for providing The RDF
  query language
• No standards as yet
• We have looked in more details at one of them,
  SeRQL, as it is an implementers language paired
  with an existing RDF repository tool, Sesame
• Many features in SeRQL remind us of SQL, thus
  learning threshold should be low

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• Next
• OWL