The Query Exchange Language

1
The Query Exchange Language
2
Edutella

• Peer-to-peer network to connect RDF models
• Query Service distributes queries and collects
  results
• Needs common query language -gt Query Exchange
  Language
• At each peer, QEL is translated to a query in the
  language used by the peers backend (e.g. SQL,
  RDQL, SeRQL, RQL, O-Telos)

3
QEL

• Essentially datalog
• Representation in RDF
• N3-like syntax
• Sample
• _at_prefix dc lthttp//purl.org/dc/elements/1.1/gt.
• _at_prefix rdfs lthttp//www.w3.org/2000/01/rdf-schem
  agt.
• label(X, L) - qels(X, rdfslabel, L).
• label(X, L) - qels(X, dctitle, L).
• ?(X,T,D)- label(X, T), qellike(T, 'RDF'),
  qels(X, dcdescription, D).

4
Built-in predicates

• RDF-specific
• qels matches triples
• qelmember matches elements of rdfSeq/Alt/Bag
• qelnodeType matches subset of nodes
  Literal, Resource, AnonymousResource
• qeldataType matches literal XML Schema datatype
• ...
• General
• qelequals
• qellike
• qellessThan, qelgreaterThan
• ...
• Negation of built-in predicates allowed

5
Outer-Join shorthand

• Often we want to get additional variable bindings
  if possible, but get results even if not all
  variables can be bound.
• Example
• ?(X,T,D)- label(X, T), qellike(T, 'RDF'),
  qels(X, dcdescription, D).
• is shorthand for
• p(X,T,D) - label(X, T), qellike(T, 'RDF'),
  qels(X, dcdescription, D).
• p(X,T,D) - label(X, T), qellike(T, 'RDF').
• ?(X,T,D)- p(X,T,D).

6
Compliance Levels

• Rule-less queries
• Conjunctive queries
• Disjunctive queries
• Linear recursive queries
• General recursive queries
• Each source provides self-description of query
  capabilities
• ltqelQELImplementation rdfabout"http//www.test.
  com/myQELimpl"gt
•     lt!-- Supports ordinary disjunctive queries.
  --gt
• ltqelsupportsComplexity rdfresource"qelDis
  junctiveQueries"/gt
• lt!-- Supports two built-in predicates. --gt
• ltqelsupportsBuiltin rdfresource"qellike"/
  gt
•     ltqelsupportsBuiltin rdfresource"qelequals
  "/gt
• lt/rdfRDFgt

7
QEL Contributions

• RDF representation
• RDF specific built-in predicates
• source capability self-description