Introduction to RDF Query with SPARQL

Dave Beckett

Yahoo! Inc

Overview

What is SPARQL?
RDF Query Languages Background
SPARQL Query Language
RDF Data in Turtle
Triple and Graph Patterns
Expressions and Values
Query Results and Data
Issues

What Is RDF?

RDF Graph := a set of RDF Triples
RDF Triple := a 3-tuple (IRI or Blank Node, IRI, IRI or Blank Node or Literal)
Blank Node := a graph-scoped identifier
IRI is Internationalized URIs (IRIs) here

What Is SPARQL?

SPARQL Protocol and RDF Query Language
(you can blame me for the cute name)

A Query Language ...:
Find names and websites of contributors to PlanetRDF:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?website
FROM   <http://planetrdf.com/bloggers.rdf>
WHERE { ?person foaf:weblog ?website ;
                foaf:name ?name .
        ?website a foaf:Document
      }

... and a Protocol.

http://.../qps?
query-lang=http://www.w3.org/TR/rdf-sparql-query/
&graph-id=http://planetrdf.com/bloggers.rdf
&query=PREFIX foaf: <http://xmlns.com/foaf/0.1/...

Run this query

Background of RDF Query Languages

RDF: W3C RECommendation February 1998
Revised RECommendations February 2004
Querying at W3C been discussed since QL'98 workshop in December 1998
Querying XML (XQuery) been developed from September 1999
Implementors have lead RDF query language development

Why do you need (RDF) query languages?

Support all of the RDF model
Knows about RDF graphs and RDF triples
Can handle RDF's semi-structured data
Supports operations on RDF graphs
To enable application development at a higher (above triple) level
To enable cross-language, cross-platform development

Pre-Existing RDF Query Language Designs

The main RDF query language styles:

SQL-like: RDQL/Squish, SeRQL, RDFDB QL, RQL, ...
XPath-like: Versa, RDFPath
Rules-like: N3QL, Triple, DQL, OWL-QL, ...
Language-like: Algae2, Fabl, Abeline
Using XML: XSLT, XPath, XQuery

Most popular by far are the SQL-like languages.

RDQL/Squish was the most popular SQL-like language with multiple independent implementations, before SPARQL.

Querying XML and Querying RDF

Concept	XML	RDF
Model	Document or Tree or Infoset (PSVI)	Set of Triples = RDF Graph
Atomic Units	Elements, Attributes, Text	Triples, URIs, Blank Nodes, Text
Identifiers	Element/Attribute names QNames IDs XPointers / XPaths	URIs
Described by	DTDs W3C XML Schema Relax NG ...	RDF Schema

XSLT / XQuery and RDF

Not based on RDF terms - graphs, URIs and literals
XQuery is based on sequences (node sets), cannot represent a set of triples
Syntax focused - so more useful for SPARQL XML output (later)
XQuery is still being developed (1999-present)

Why standardize RDF query now?

given that RDQL (and others) are widely implemented

Querying RDF has been discussed since 1998
Implementations of RDQL are similar but have some divergence and extensions
Recent research with contexts and Named Graphs are not in all current RDF query languages
XML query solutions exist but do not entirely suit RDF.

So in March 2004 the W3C formed the RDF Data Access Working Group

It has just reached Last Call stage February 2006

Use Cases for RDF Query

Some of the use cases DAWG recorded were:

Finding values for partially known graph structures
Getting information about an identifiable object with unknown properties
A human friendly syntax for queries for application developers
Running automated regular queries against RDF graphs
Querying aggregated RDF graphs
Running queries constrained with datatype expressions
Querying a remote RDF server and getting streaming results back
Allowing alternate solutions to match in queries
Using local extension functions in a query
Using an RDF query service with Web Services

Requirements for RDF query

These led to requirements

from existing languages:: conjunction (AND) of triple patterns with variable bindings and constraints
from use cases:: graphs, datatypes, extension functions, aggregation, alternates, descriptions
from the WG charter:: a protocol.
no rules language, no cursors, no proofs and no updates [more of this later]

SPARQL - Query Language

An RDF data access query language
Data access means reading information, not writing (updates)
Outline query model is graph patterns - a data graph with constants replaced with variable names
In style of earlier RDQLish work - i.e. not rules or path based

SPARQL query syntax

Make query look like the data with variables substituted
Turtle / N3 data style chosen (eventually!)
Generalise RDF triples to be 3-array of RDF Terms
RDF Term := RDF IRI | Blank Node | Literal
Triple Pattern := 3-array of (RDF Term | Variable Name)
(Consequence: you can match triples that RDF does not support)

plus more abbreviating syntax

Turtle RDF syntax - IRIs and Blank Nodes

IRIs: Enclosed in <>
Relative IRI references turned into IRIs
<IRI>; or
@prefix prefix <http://....>
prefix:name
in the style of XML QNames as a shorthand for the full IRI
Blank Nodes: _:name; or
[] for a Blank Node used once

Turtle RDF syntax - RDF Literals

Literals

"Literal"

"Literal"@language

"""Long literal with
newlines"""

Datatyped Literals

`"`lexical form`"^^`datatype IRI e.g. `"10"^^xsd:integer`
`10`	Decimal integer (`xsd:integer`)
`true`	Boolean (`xsd:boolean`)
`2.5`	Decimal (`xsd:decimal`)

Turtle RDF Syntax - Abbreviations

Triples separated by .

:a :b :c . :d :e :f .

Common triple predicate and subject:

:a :b :c ,
      :d .

which is the same as

:a :b :c .
:a :b :d .

Common triple subject:

:a :b :c ;
   :d :e .

which is the same as:

:a :b :c .
:a :d :e .

Turtle RDF Syntax - Abbreviations 2

Blank node as a subject

:a :b [ :c :d ] which is the same as:

:a :b _:x .
_:x :c :d .

for blank node _:x

RDF Collections

:a :b ( :c :d :e :f )
which is short for many many triples :)

SPARQL Triple Patterns

Add variables to a Turtle RDF graph. Data:

:a :b :c .
:c :d "hello"

Query: Find me the x in the RDF graph such that this matches:

?x :b :c .
:c :d "hello"

The full SPARQL query:

PREFIX : <http://example.org/stuff#>
SELECT ?v
WHERE {
  ?v :b :c .
  :c :d "hello"
}

{ } is a SPARQL Graph Pattern

SPARQL Graph Patterns (GPs)

Basic: contains only triple patterns which must all match
Group: contain GPs that MUST match or the GP fails
Optional: contain GPs that MAY match
Union: contain alternate GPs, any or all of which MAY match

All may contain FILTER expressions.

FILTER Expressions and Values

Graph patterns can be constrained by FILTER expressions
Expressions over bound variables returning True, False or error
Arithmetic logical operators, functions, type promotion, casting
Extension functions my:function
XSD Datatypes: boolean, integer, decimal, dateTime

FILTER ?x > 3
FILTER BOUND(?x)
FILTER (?date < "2006-03-01T20:15:00Z"^^xsd:date)

Query Results

Like SQL - a table; a sequence of rows of variable bindings
Aligns with existing SQL support
- Protocols such as ODBC and JDBC
- SQL Database access APIs such as Perl DBI
- SPARQL has been added inside Oracle and (maybe?) PostgreSQL
OPTIONAL can give unbound variables. This is not SQL NULL
SELECT DISTINCT on a per-row basis
ORDER BY: ascending or descending sort order
Result sub-sequences LIMIT and OFFSET (not cursors)

Data Management

Specify input graphs with FROM

FROM <http://planetrdf.com/bloggers.rdf>

Give them names with FROM NAMED

FROM NAMED <http://site1.example.com/foo.rdf>
FROM NAMED <http://site2.example.com/bar.rdf>

Get the names back with GRAPH ?g GP

WHERE { GRAPH ?g { ?s dc:title "Der Baum"@de } }

Model is 1 background graph + many Named Graphs

The Rest

RDF Blank Nodes in a query act as anonymous variables
ASK: ask if there is any answer, do not return results
- sometimes you want to know there is an answer but don't care what it is
- sometimes you don't know what it is
DESCRIBE: tell me about a named thing
CONSTRUCT: Build an RDF graph from substituting bindings into a second graph pattern. A 1-step rule language

Use Every SPARQL Keyword Example



BASE <http://example.org/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
# This is a relative IRI to BASE above
PREFIX ex: <properties/1.0#>

SELECT DISTINCT $person ?name $age
FROM <http://rdf.example.org/personA.rdf>
FROM <http://rdf.example.org/personB.rdf>
WHERE {  $person a foaf:Person ;
                 foaf:name ?name.
         OPTIONAL { $person ex:age $age } .
         FILTER (!REGEX(?name, "Bob"))
      }
ORDER BY ASC(?name) LIMIT 10 OFFSET 20

Rasqal SPARQL examples

SPARQL Status

At Last Call stage February 2006
Several mature query language implementations: SPARQL implementations
3+ in Java alone
Oracle developing support
Completing shortly

Extending SPARQL Query

Extension functions:

WHERE { ...
  FILTER my:function(?a, ?b, 25)
}

"Magic" predicates overload meaning:
```
WHERE { ...
  ?a xsparql:closeto ?b
}
```
(only for 2-ary predicates)
Use conventions of graph patterns to mean something special
True language extensions: new keywords:
```
WHERE { ?a CLOSETO(5) ?b }
```

SPARQL - Protocol

Services running SPARQL queries over a set of graphs
A transport protocol for invoking the service
Based on ideas from earlier protocol work such as Joseki
Describing the service with Web Service technologies

SPARQL Issues

No update. Reason: out of charter
No cursors. Reason: out of charter, not a stateful protocol
No computed results like SELECT (?x+?y) WHERE ...
No aggregate functions like SELECT COUNT(?x) WHERE ...
No fulltext search support
No support for querying RDF collections
- A problem for OWL as many OWL concepts are encoded in rdf:first / rdf:rest lists
- Could be something like ?item IN ?list
- No good design emerged
Cannot query variable length paths (Versa)
Expression functionality does not include all datatypes
Missing datatype functions/operators critical for some people

Thanks

Questions?

Slides (will be at):
http://www.dajobe.org/talks/200603-sparql-stanford
(and the CSS will be better too)

Rasqal SPARQL demo
http://librdf.org/query

Introduction to RDF Query with SPARQL

Copyright Yahoo! Inc 2006

Introduction to RDF Query with SPARQL

Dave Beckett

Yahoo! Inc

Overview

What Is RDF?

What Is SPARQL?

Background of RDF Query Languages

Why do you need (RDF) query languages?

Pre-Existing RDF Query Language Designs

Querying XML and Querying RDF

XSLT / XQuery and RDF

Why standardize RDF query now?

Use Cases for RDF Query

Requirements for RDF query

SPARQL - Query Language

SPARQL query syntax

Turtle RDF syntax - IRIs and Blank Nodes

Turtle RDF syntax - RDF Literals

Turtle RDF Syntax - Abbreviations

Turtle RDF Syntax - Abbreviations 2

SPARQL Triple Patterns

SPARQL Graph Patterns (GPs)

FILTER Expressions and Values

Query Results

Data Management

The Rest

Use Every SPARQL Keyword Example

SPARQL Status

Extending SPARQL Query

SPARQL - Protocol

SPARQL Issues

Thanks

Questions?

Copyright