Semantic Web - Spring 2006
Computer Engineering Department
Sharif University of Technology

• XQuery
– Querying on XML Data
• RDQL
– Querying on RDF Data
• SparQL
– Another RDF query language (under development)
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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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• 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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• 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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<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>
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Part to be returned as it is given {To be evaluated}
<titles> doc( String ) returns input document
{for $r in doc("recipes.xml")//recipe return
Iteration $var - variables
$r/title}
</titles>
XPath
Sequence of results, one for each variable binding
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• 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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• doc("recipes.xml")//recipe[1]/title returns
<title>Beef Parmesan with Garlic Angel Hair Pasta</title> an element
• 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> a list of elements
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• doc("recipes.xml")//recipe[1]/ingredient[1]
/@name
→ attribute name {"beef cube steak"} a constructor for an attribute node
• string(doc("recipes.xml")//recipe[1]
/ingredient[1]/@name)
→ "beef cube steak" a value of type string
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• <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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• <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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• <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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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")//recipe for $v in doc("vegetables.xml")//vegetable return ...
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<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>
Returns my-recipes with titles as attributes and my-ingredients with names as text content
}
</my-recipes>
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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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<calory-content>
{let $ooreps := doc("recipes.xml")//recipe
[.//ingredient/@name="olive oil"] for $r in $ooreps return
<calories> Calories of recipes
{$r/title/text()}
{": "} with olive oil
{string($r/nutrition/@calories)}
</calories> }
</calory-content>
Note the implicit string concatenation
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The query returns:
<calory-content>
<calories>Beef Parmesan: 1167</calories>
<calories>Linguine Pescadoro: 532</calories>
</calory-content>
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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")//recipe where $r//ingredient/@name="olive oil" return ...
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• Syntax: some / every $ satisfies var in
<node-set>
<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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• Recipes that have some compound ingredient for $r in doc("recipes.xml")//recipe where some $i in $r/ingredient satisfies $i/ingredient
Return $r/title
• Recipes where every ingredient is non-compound for $r in doc("recipes.xml")//recipe where every $i in $r/ingredient satisfies not($i/ingredient)
Return $r/title
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“To every recipe, add the attribute calories!”
<result>
{let $rs := doc("recipes.xml")//recipe for $r in $rs return
<recipe>
{$r/nutrition/@calories} an attribute
{$r/title}
</recipe> } an element
</result>
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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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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 $rs := doc("recipes.xml")//recipe return distinct-values($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",
...
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(cntd.)
The interpreter must be told whether the values should be regarded as numbers or as strings
(alphanumerical sorting is default) for $r in $rs order by number($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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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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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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“The recipe with the maximal number of calories!” let $rs := doc("recipes.xml")//recipe let $maxCal := max($rs//@calories) for $r in $rs where $r//@calories = $maxCal return string($r/title) returns
"Cailles en Sarcophages"
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• Galax is an open-source implementation of
XQuery ( http://www.galaxquery.org/ )
– The main developers have taken part in the definition of
XQuery
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Querying on RDF data

• R DF D ata Q uery L anguage
• JDBC/ODBC friendly
• Simple:
SELECT some information
FROM somewhere
WHERE this match
AND these constraints
USING these vocabularies
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• 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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• Return all the resources that have property FN and the associated values:
SELECT ?x, ?fname
WHERE (?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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• 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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• 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 ?givenName
WHERE (?y, vCard:Family, "Smith"),
(?y, vCard:Given, ?givenName)
USING vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#>
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• 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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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)
USING isc 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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• Jena
– http://jena.sourceforge.net/
• Sesame
– http://sesame.aidministrator.nl/
• RDFStore
– <http://rdfstore.sourceforge.net/>
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• Does not take into account semantics of RDFS
• For example: ex:human rdfs:subClassOf ex:animal ex:student rdfs:subClassOf ex:human ex:john rdf:type ex:student
Query: “ To which class does the resource John belong?”
Expected answer: ex:student, ex:human, ex:animal
However, the query:
SELECT ?x
WHERE (<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
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• 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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• 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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• 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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• 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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• 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/
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