==== ARQ 2.1

ARQ 2.1 is a complete implementation of SPARQL including
support for custom filter function, property functions
and free text search.  It also provides an update language
based on SPARQL.

== New in ARQ 2.1

  * Cost-based Optimizer
  * GROUP BY
  * Counting
  * SERVICE - call-out to another query service 
  * Expressions in the SELECT clause 

== Cost-based Optimizer 

The ARQ-2.1 release includes a static cost-based optimizer for
Basic Graph Patterns for in-memory models. The optimizer uses heuristics
to estimate the execution cost of triple patterns and reorders the patterns
of a BGP according to their estimated costs. The optimizer is enabled by
default in ARQ but may be disabled if this should be required.

Please refer to the documentation for further information.
http://jena.sourceforge.net/ARQ/bgp-optimization.html

== GROUP BY

Support for GROUP BY in SELECT queries.  This groups pattern solutions by common
variables, and applies aggregate functions to the solutiosn with common group
variables.  Counting is provided as an aggregate function.   

A HAVING clause allows a filer to be applied after the group operation

== Counting

Aggreggeate function over a GROUP BY query.  Aggregate functiosn are uually name
with AS using the syntax of SELECT expressions.  

Supported: count(*), count(?x), count(distinct *), count(distinct ?x)  

Examples:

  SELECT (count(*) AS ?count)
  { pattern } 

will return the number of matches
of a pattern. 

  SELECT ?p (count(?q) AS ?num_q)
  {
      ?x a :p .
      ?x :p ?p .
      ?x :q ?q .
  } GROUP BY ?p

== Remote query support

The ARQ query language, an extension of SPARQL, includes the ability
to call a remote SPARQL service using the SERVICE form.  

  SERVICE <serviceEndPoint> { pattern }
  
The pattern is sent to and executed at the remote service.  ARQ combines the
results with the locally part of the query.  The algebra supports a new
operation, OpService, for this.  Algebra expression can be turned back into
SPARQL query syntax.

== SELECT expressions

Expressions, as well as plain variables, can be given in the SELECT clause.

Example: SELECT (?x+?y AS ?z)

Use of AS is not required but is strongly encouraged.

== Links

Download:
  http://prdownloads.sourceforge.net/jena/ARQ-2.1.zip
  Use jena.jar in this download - ARQ does not work with Jena.2.5.3

Home page:
  http://jena.sourceforge.net/ARQ/

ARQ is also available via anonymous SVN:
  https://jena.svn.sourceforge.net/svnroot/jena/ARQ/

SPARQL is described in:
  http://www.w3.org/TR/rdf-sparql-query/
  
SPARQL-Update is described in:
  http://jena.hpl.hp.com/~afs/SPARQL-Update.html

== Status

The implementation of SPARQL is complete and meets the
SPARQL current editors' working draft and Candidate Recommendation.

ARQ passes all the DAWG tests.

Latest published:
  http://www.w3.org/TR/rdf-sparql-query/

Editors' working draft:
  http://www.w3.org/2001/sw/DataAccess/rq23/rq25.html

Definition of SPARQL:
  http://www.w3.org/2001/sw/DataAccess/rq23/rq25.html#sparqlDefinition

== Notes

+ The old ARQ-1 query engine "engine1" has been removed.

== Online Services

SPARQL syntax validator: 
  http://www.sparql.org/validator.html

SPARQL processor:
  http://www.sparql.org/sparql.html

== Support and Questions

Comments and questions about ARQ to the Jena mailing list 
    jena-dev@groups.yahoo.com

General questions and comments on SPARQL to jena-dev or
    public-sparql-dev@w3.org
    
Formal comments specific to the SPARQL query language,
protocol or result set documents to:
	public-rdf-dawg-comments@w3.org