Querying on the Web: XQuery, RDQL, SparQL Semantic Web - Spring 2006 Computer Engineering Department Sharif University of Technology

2 Outline XQuery –Querying on XML Data RDQL –Querying on RDF Data SparQL –Another RDF query language (under development)

3 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

4 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

5 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

6 Sample data

7 {for $r in doc("recipes.xml")//recipe return $r/title} returns Beef Parmesan with Garlic Angel Hair Pasta Ricotta Pie … A Query over the Recipes Document

8 XPath {for $r in doc("recipes.xml")//recipe return $r/title} 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

9 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

10 XPath is a Fragement of XQuery doc("recipes.xml")//recipe[1]/title returns Beef Parmesan with Garlic Angel Hair Pasta doc("recipes.xml")//recipe[position()<=3] /title returns Beef Parmesan with Garlic Angel Hair Pasta, Ricotta Pie, Linguine Pescadoro an element a list of elements

11 Beware: XPath Attributes doc("recipes.xml")//recipe[1]/ingredient[1] attribute name {"beef cube steak"} string(doc("recipes.xml")//recipe[1] "beef cube steak" a constructor for an attribute node a value of type string

12 XPath Attributes (cntd.) {string(doc("recipes.xml")//recipe[1] beef cube steak an element with string content

13 XPath Attributes (cntd.) {doc("recipes.xml")//recipe[1] an element with an attribute

14 XPath Attributes (cntd.) <first-ingredient oldName="{doc("recipes.xml")//recipe[1] Beef Beef An attribute is cast as a string

15 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")//recipe for $v in doc("vegetables.xml")//vegetable return...

16 Nested For-clauses: Example {for $r in doc("recipes.xml")//recipe return {for $i in $r//ingredient return } } Returns my-recipes with titles as attributes and my-ingredients with names as text content

17 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 oil"]

18 Let Clause: Example {let $ooreps := doc("recipes.xml")//recipe oil"] for $r in $ooreps return {$r/title/text()} {": "} } Calories of recipes with olive oil Note the implicit string concatenation

19 Let Clause: Example (cntd.) The query returns: Beef Parmesan: 1167 Linguine Pescadoro: 532

20 The Where Clause Syntax: where occurs before return clause similar to predicates in XPath comparisons on nodes: –" = " for node equality –" > " for document order Example: for $r in doc("recipes.xml")//recipe where oil" return...

21 Quantifiers Syntax: some / every $ var in satisfies $ var is bound to all nodes in Test succeeds if is true for some/every binding Note: if is empty, then some is false and all is true

22 Quantifiers (Example) Recipes that have some compound ingredient Recipes where every ingredient is non-compound for $r in doc("recipes.xml")//recipe where some $i in $r/ingredient satisfies $i/ingredient Return $r/title for $r in doc("recipes.xml")//recipe where every $i in $r/ingredient satisfies not($i/ingredient) Return $r/title

23 Element Fusion To every recipe, add the attribute calories! {let $rs := doc("recipes.xml")//recipe for $r in $rs return {$r/title} } an elementan attribute

24 Element Fusion (cntd.) The query result: Beef Parmesan with Garlic Angel Hair Pasta Ricotta Pie Linguine Pescadoro

25 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")//recipe return yields "beef cube steak onion, sliced into thin rings...

26 Syntax: order by expr [ ascending | descending ] for $iname in order by $iname descending return string($iname) yields "whole peppercorns", "whole baby clams", "white sugar",... The Order By Clause

27 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 $rs order by 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!

28 Grouping and Aggregation Aggregation functions count, sum, avg, min, max Example: The number of simple ingredients per recipe for $r in doc("recipes.xml")//recipe return {attribute {"title"} {$r/title/text()}} {count($r//ingredient[not(ingredient)])}

29 Grouping and Aggregation (cntd.) The query result: 11, 12, 15, 8, 30

Nested Aggregation The recipe with the maximal number of calories! let $rs := doc("recipes.xml")//recipe let $maxCal := for $r in $rs where = $maxCal return string($r/title) returns "Cailles en Sarcophages"

31 Running Queries with Galax Galax is an open-source implementation of XQuery ( –The main developers have taken part in the definition of XQuery

RDQL Querying on RDF data

33 Introduction RDF Data Query Language JDBC/ODBC friendly Simple: SELECT some information FROM somewhere WHERE this match AND these constraints USING these vocabularies

34 Example

35 Example q1 contains a query: SELECT ?x WHERE (?x,, "John Smith") For executing q1with a model m1.rdf: java jena.rdfquery --data m1.rdf --query q1 The outcome is: x =============================

36 Example Return all the resources that have property FN and the associated values: SELECT ?x, ?fname WHERE (?x,, ?fname) The outcome is: x | fname ================================================ | "John Smith" | "Sarah Jones" | "Matt Jones"

37 Example Return the first name of Jones: SELECT ?givenName WHERE (?y,, "Jones"), (?y,, ?givenName) The outcome is: givenName ========= "Matthew" "Sarah"

38 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 SELECT ?givenName WHERE (?y, vCard:Family, "Smith"), (?y, vCard:Given, ?givenName) USING vCard FOR

39 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

40 Another Example SELECT ?title ?description ?orbit ?satellite ?sensor ?date FROM WHERE (?item ?title) (?item ?description) (?item ?orbit) (?item ?satellite) (?item ?sensor) (?item ?date) USING isc FOR dc FOR rdf FOR rdfs FOR

41 Implementations Jena – Sesame – RDFStore –

42 Limitation 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 (, rdf:type, ?x) USING rdf FOR Yields only: Solution: Inference Engines

SparQL

44 Introduction A RDF query language currently under development by W3C Builds on previous RDF query languages such as rdfDB, RDQL, and SeRQL.

45 Example RDF

46 Example Simple Query: PREFIX foaf: SELECT ?url FROM WHERE { ?contributor foaf:name "Jon Foobar". ?contributor foaf:weblog ?url. }

47 Example (cont.) Optional block: PREFIX foaf: SELECT ?name ?depiction WHERE { ?person foaf:name ?name. OPTIONAL { ?person foaf:depiction ?depiction. } }

48 Example (cont.) Alternative matches: PREFIX foaf: PREFIX rdf: 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.

49 References A Programmer's Introduction to RDQL –