Querying on the Web:XQuery, RDQL, SparQL

Semantic Web - Spring 2006

Computer Engineering Department

Sharif University of Technology

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Outline

• XQuery– Querying on XML Data

• RDQL– Querying on RDF Data

• SparQL– Another RDF query language (under development)

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Requirements for an XML Query Language

David Maier, W3C XML Query Requirements:• Closedness: output must be XML• Composability: wherever a set of XML elements is

required, a subquery is allowed as well• Can benefit from a schema, but should also be applicable

without• Retains the order of nodes• Formal semantics

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How Does One Design a Query Language?

• In most query languages, there are two aspects to

a query:

– Retrieving data (e.g., from … where … in SQL)

– Creating output (e.g., select … in SQL)

• Retrieval consists of

– Pattern matching (e.g., from … )

– Filtering (e.g., where … )

… although these cannot always be clearly distinguished

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XQuery Principles

• A language for querying XML document.

• Data Model identical with the XPath data model– documents are ordered, labeled trees

– nodes have identity

– nodes can have simple or complex types (defined in XML Schema)

• XQuery can be used without schemas, but can be checked against DTDs and XML schemas

• XQuery is a functional language– no statements

– evaluation of expressions

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Sample data

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<titles>

{for $r in doc("recipes.xml")//recipe

return $r/title}

</titles>

returns

<titles>

<title>Beef Parmesan with Garlic Angel Hair Pasta</title>

<title>Ricotta Pie</title>

…

</titles>

A Query over the Recipes Document

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XPath

<titles>

{for $r in doc("recipes.xml")//recipe

return

$r/title}

</titles>

Query Features

doc(String) returns input document

Part to be returned as it is given {To be evaluated}

Iteration $var - variables

Sequence of results,one for each variable binding

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Features: Summary

• The result is a new XML document

• A query consists of parts that are returned as is

• ... and others that are evaluated (everything in {...} )

• Calling the function doc(String) returns an input document

• XPath is used to retrieve nodes sets and values

• Iteration over node sets:

let binds a variable to all nodes in a node set

• Variables can be used in XPath expressions

• return returns a sequence of results,

one for each binding of a variable

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XPath is a Fragement of XQuery• doc("recipes.xml")//recipe[1]/title

returns

<title>Beef Parmesan with Garlic Angel Hair Pasta</title>

• doc("recipes.xml")//recipe[position()<=3] /title

returns

<title>Beef Parmesan with Garlic Angel Hair Pasta</title>,

<title>Ricotta Pie</title>,

<title>Linguine Pescadoro</title>

an element

a list of elements

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Beware: XPath Attributes

• doc("recipes.xml")//recipe[1]/ingredient[1] /@name

→ attribute name {"beef cube steak"}

• string(doc("recipes.xml")//recipe[1] /ingredient[1]/@name)

→ "beef cube steak"

a constructor for an attribute node

a value of type string

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XPath Attributes (cntd.)

• <first-ingredient>{string(doc("recipes.xml")//recipe[1] /ingredient[1]/@name)}</first-ingredient>

→ <first-ingredient>beef cube steak</first-ingredient>

an element with string content

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XPath Attributes (cntd.)

• <first-ingredient>{doc("recipes.xml")//recipe[1] /ingredient[1]/@name}

</first-ingredient>

→ <first-ingredient name="beef cube steak"/>

an element with an attribute

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XPath Attributes (cntd.)

• <first-ingredient

oldName="{doc("recipes.xml")//recipe[1] /ingredient[1]/@name}">Beef</first-ingredient>

→ <first-ingredient oldName="beef cube steak">

Beef

</first-ingredient>

An attribute is cast as a string

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Iteration with the For-Clause

Syntax: for $var in xpath-expr

Example: for $r in doc("recipes.xml")//recipe return string($r)

• The expression creates a list of bindings for a variable $var

If $var occurs in an expression exp,

then exp is evaluated for each binding

• For-clauses can be nested:

for $r in doc("recipes.xml")//recipefor $v in doc("vegetables.xml")//vegetable return ...

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Nested For-clauses: Example

<my-recipes>

{for $r in doc("recipes.xml")//recipe

return

<my-recipe title="{$r/title}">

{for $i in $r//ingredient

return

<my-ingredient>

{string($i/@name)}

</my-ingredient>

}

</my-recipe>

}

</my-recipes>

Returns my-recipes with titles as attributes and my-ingredientswith names as text content

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The Let Clause

Syntax: let $var := xpath-expr

• binds variable $var to a list of nodes,

with the nodes in document order

• does not iterate over the list

• allows one to keep intermediate results for reuse

(not possible in SQL)

Example:

let $ooreps := doc("recipes.xml")//recipe

[.//ingredient/@name="olive oil"]

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Let Clause: Example

<calory-content>

{let $ooreps := doc("recipes.xml")//recipe

[.//ingredient/@name="olive oil"]

for $r in $ooreps return

<calories>

{$r/title/text()}

{": "}

{string($r/nutrition/@calories)}

</calories>}

</calory-content>

Calories of recipeswith olive oil

Note the implicitstring concatenation

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Let Clause: Example (cntd.)

The query returns:

<calory-content>

<calories>Beef Parmesan: 1167</calories>

<calories>Linguine Pescadoro: 532</calories>

</calory-content>

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The Where Clause

Syntax: where <condition>• occurs before return clause • similar to predicates in XPath• comparisons on nodes:

– "=" for node equality– "<<" and ">>" for document order

• Example:

for $r in doc("recipes.xml")//recipewhere $r//ingredient/@name="olive oil"return ...

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Quantifiers

• Syntax: some/every $var in <node-set> satisfies <expr>

• $var is bound to all nodes in <node-set> • Test succeeds if <expr> is true for some/every

binding• Note: if <node-set> is empty, then

“some” is false and “all” is true

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Quantifiers (Example)

• Recipes that have some compound ingredient

• Recipes where every ingredient is non-compound

for $r in doc("recipes.xml")//recipewhere some $i in $r/ingredient satisfies $i/ingredient Return $r/title

for $r in doc("recipes.xml")//recipewhere every $i in $r/ingredient satisfies not($i/ingredient) Return $r/title

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Element Fusion

“To every recipe, add the attribute calories!”<result>

{let $rs := doc("recipes.xml")//recipe

for $r in $rs return

<recipe>

{$r/nutrition/@calories}

{$r/title}

</recipe>}

</result>

an element

an attribute

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Element Fusion (cntd.)

The query result:

<result>

<recipe calories="1167">

<title>Beef Parmesan with Garlic Angel Hair Pasta</title>

</recipe>

<recipe calories="349">

<title>Ricotta Pie</title>

</recipe>

<recipe calories="532">

<title>Linguine Pescadoro</title>

</recipe>

</result>

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Eliminating Duplicates

The function distinct-values(Node Set)

– extracts the values of a sequence of nodes

– creates a duplicate free sequence of values

Note the coercion: nodes are cast as values!

Example:

let $rs := doc("recipes.xml")//recipereturn distinct-values($rs//ingredient/@name)

yields

"beef cube steak

onion, sliced into thin rings

...

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Syntax: order by expr [ ascending | descending ]

for $iname in doc("recipes.xml")//@name

order by $iname descending

return string($iname)

yields

"whole peppercorns",

"whole baby clams",

"white sugar",

...

The Order By Clause

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The Order By Clause (cntd.)

The interpreter must be told whether the values should be regarded as numbers or as strings (alphanumerical sorting is default)

for $r in $rsorder by number($r/nutrition/@calories)return $r/title

Note:

– The query returns titles ...

– but the ordering is according to calories, which do not appear in the output

Not possible in SQL!

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Grouping and Aggregation

Aggregation functions count, sum, avg, min, max

Example: The number of simple ingredients

per recipe

for $r in doc("recipes.xml")//recipe

return

<number>

{attribute {"title"} {$r/title/text()}}

{count($r//ingredient[not(ingredient)])}

</number>

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Grouping and Aggregation (cntd.)

The query result:

<number title="Beef Parmesan with Garlic Angel Hair Pasta">11</number>,

<number title="Ricotta Pie">12</number>,

<number title="Linguine Pescadoro">15</number>,

<number title="Zuppa Inglese">8</number>,

<number title="Cailles en Sarcophages">30</number>

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Nested Aggregation

“The recipe with the maximal number of calories!”

let $rs := doc("recipes.xml")//recipelet $maxCal := max($rs//@calories)for $r in $rswhere $r//@calories = $maxCalreturn string($r/title)

returns

"Cailles en Sarcophages"

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Running Queries with Galax

• Galax is an open-source implementation of

XQuery (http://www.galaxquery.org/)

– The main developers have taken part in the definition of

XQuery

RDQL

Querying on RDF data

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Introduction

• RDF Data Query Language• JDBC/ODBC friendly

• Simple:

SELECTsome information

FROMsomewhere

WHEREthis match

ANDthese constraints

USINGthese vocabularies

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Example

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Example

• q1 contains a query:SELECT ?x

WHERE (?x, <http://www.w3.org/2001/vcard-rdf/3.0#FN>, "John Smith")

• For executing q1with a model m1.rdf:java jena.rdfquery --data m1.rdf --query q1

• The outcome is:x

=============================

<http://somewhere/JohnSmith/>

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Example

• Return all the resources that have property FN and the associated values:

SELECT ?x, ?fnameWHERE (?x, <http://www.w3.org/2001/vcard-rdf/3.0#FN>, ?fname)

• The outcome is:

x | fname ================================================<http://somewhere/JohnSmith/> | "John Smith" <http://somewhere/SarahJones/> | "Sarah Jones"<http://somewhere/MattJones/> | "Matt Jones"

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Example

• Return the first name of Jones:

SELECT ?givenName

WHERE (?y, <http://www.w3.org/2001/vcard-rdf/3.0#Family>, "Jones"),

(?y, <http://www.w3.org/2001/vcard-rdf/3.0#Given>, ?givenName)

• The outcome is:

givenName

=========

"Matthew"

"Sarah"

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URI Prefixes : USING

• RDQL has a syntactic convenience that allows prefix strings to be defined in the USING clause :

SELECT ?x WHERE (?x, vCard:FN, "John Smith") USING vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#>

SELECT ?givenNameWHERE (?y, vCard:Family, "Smith"),

(?y, vCard:Given, ?givenName) USING vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#>

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Filters

• RDQL has a syntactic convenience that allows prefix strings to be defined in the USING clause :

SELECT ?resource WHERE (?resource, info:age, ?age) AND ?age >= 24 USING info FOR <http://somewhere/peopleInfo#>

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Another Example

SELECT?title ?description ?orbit ?satellite ?sensor ?date

FROM<http://earth.esa.int/showcase/ers/dublin.rdf>

WHERE(?item <dc:title> ?title)(?item <dc:description> ?description)(?item <isc:orbit> ?orbit)(?item <isc:satellite> ?satellite)(?item <isc:sensor> ?sensor)(?item <dc:date> ?date)

USINGisc FOR <http://earth.esa.int/standards/showcase/>dc FOR <http://purl.org/dc/elements/1.1/>rdf FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>rdfs FOR <http://www.w3.org/2000/01/rdf-schema#>

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Implementations

• Jena– http://jena.sourceforge.net/

• Sesame– http://sesame.aidministrator.nl/

• RDFStore– <http://rdfstore.sourceforge.net/>

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Limitation

• Does not take into account semantics of RDFS• For example:

ex:human rdfs:subClassOf ex:animalex:student rdfs:subClassOf ex:humanex:john rdf:type ex:student

Query: “ To which class does the resource John belong?”Expected answer: ex:student, ex:human, ex:animalHowever, the query:

SELECT ?xWHERE (<http://example.org/#john>, rdf:type, ?x)USING rdf FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

Yields only:<http://example.org/#student>

• Solution: Inference Engines

SparQL

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Introduction

• A RDF query language currently under development by W3C

• Builds on previous RDF query languages such as rdfDB, RDQL, and SeRQL.

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Example RDF

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Example

• Simple Query:

PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT ?url FROM <bloggers.rdf> WHERE {

?contributor foaf:name "Jon Foobar" . ?contributor foaf:weblog ?url . }

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Example (cont.)

• Optional block:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>

SELECT ?name ?depiction

WHERE { ?person foaf:name ?name .

OPTIONAL { ?person foaf:depiction ?depiction . }

}

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Example (cont.)

• Alternative matches:

PREFIX foaf: <http://xmlns.com/foaf/0.1/> PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> SELECT ?name ?mbox WHERE { ?person foaf:name ?name . { { ?person foaf:mbox ?mbox } UNION { ?person foaf:mbox_sha1sum ?mbox } } }

• There are many other features in SparQL which is out of scope for this class. Refer to references for more information.

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References

• http://www.w3.org/TR/xquery/

• A Programmer's Introduction to RDQL– http://jena.sourceforge.net/tutorial/RDQL/

• http://rdfstore.sourceforge.net/

• http://jena.sourceforge.net

• http://sesame.aidministrator.nl/

• http://www.w3.org/TR/2004/WD-rdf-sparql-query-20041012/

• http://www-128.ibm.com/developerworks/java/library/j-sparql/