Semantic Web. Querying on the Web: XQuery, RDQL, SparQL. Morteza Amini. Sharif University of Technology Fall 94-95

Transcription

1 ه عا ی Semantic Web Querying on the Web: XQuery, RDQL, SparQL Morteza Amini Sharif University of Technology Fall 94-95

2 Outline XQuery Querying on XML Data RDQL Querying on RDF Data SparQL Another RDF query language 2

3 Outline XQuery Querying on XML Data RDQL SparQL 3

4 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 4

5 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

6 XQuery Principles XQuery is a language for querying XML documents. It is built on Xpath expressions and its data model is identical with the XPath data model: documents are ordered, labelled 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 XQuery for XML is like SQL for databases. XQuery is supported by all major DBMSs. 6

7 Sample XML Structure <?xml version= 1.0 > <collection xmlns:= xmlns:xsi= > <description>.. </description> <recipe> <title> </title> <ingredient name= amount= unit= calories= /> <ingredient name= amount= unit= calories= /> <preparation> <step>. </step> <step>. </step> </preparation> <nutrition calories= /> <comment>. </comment> </recipe> <recipe> </recipe> </collection> 7

8 Sample XML Data 8

9 A Query over the Recipes Document <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> 9

10 Query Features 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 $r/title} Iteration $var - variables XPath </titles> Sequence of results, one for each variable binding 10

11 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: 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. 11

12 XPath is a Fragement of XQuery 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 12

13 Beware: XPath Attributes doc("recipes.xml")//recipe[1]/ingredient[1] 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 13

14 XPath Attributes (2) <first-ingredient> {string(doc("recipes.xml")//recipe[1] </first-ingredient> <first-ingredient>beef cube steak</first-ingredient> an element with string content 14

15 XPath Attributes (3) <first-ingredient> {doc("recipes.xml")//recipe[1] </first-ingredient> <first-ingredient name="beef cube steak"/> an element with an attribute 15

16 XPath Attributes (4) <first-ingredient oldname="{doc( recipes.xml )//recipe[1] Beef </first-ingredient> <first-ingredient oldname="beef cube steak"> Beef </first-ingredient> An attribute is cast as a string 16

17 Conditional Expressions Syntax: if (condition) then expr else expr Parentheses around the if expression are required. else is required, but it can be just else () Example: for $r in doc("recipes.xml")//recipe return if oil") then $r//title else () 17

18 Comparisons Two ways of comparing values. General comparisons: =,!=, <, <=, >, >= Value comparisons: eq, ne, lt, le, gt, ge Difference is as follows: > 300) returns true if any calories attributes have a value greater than 300. ($r//nutrition/@calories gt 300) returns true if there is only one calories attribute returned by the expression and its value is greater than 300. If more than one is returned, an error occurs. 18

19 Iteration with the For-Clause Syntax: for $var in exp Example: for $r in doc("recipes.xml")//recipe return ($r/title) The expression exp creates a list of bindings for a variable $var For-clauses can be nested: for $r in doc("recipes.xml")//recipe for $v in doc("vegetables.xml")//vegetable return... 19

20 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> </my-ingredient> } </my-recipe> } </my-recipes> Returns my-recipes with titles as attributes and my-ingredients with names as text content 20

21 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. Example: let $ooreps := doc("recipes.xml")//recipe [.//ingredient/@name="olive oil"] 21

22 Let Clause: Example <calory-content> {let $ooreps := doc("recipes.xml")//recipe oil"] for $r in $ooreps return <calories> {$r/title/text()} {": "} </calories>} </calory-content> Calories of recipes with olive oil Note the implicit string concatenation 22

23 Let Clause: Example (cntd.) The query returns: <calory-content> <calories>beef Parmesan: 1167</calories> <calories>linguine Pescadoro: 532</calories> </calory-content> 23

24 The Where Clause Syntax: where <condition> occurs before return clause similar to predicates in XPath Example: for $r in doc("recipes.xml")//recipe where oil" return... 24

25 The Order By Clause Syntax: order by expr [ascending descending] Defines the sort-order. Example: for $iname in order by $iname descending return string($iname) yields "whole peppercorns", "whole baby clams", "white sugar",... 25

26 The Order By Clause (cntd.) for $r in doc("recipes.xml")//recipe order by return $r/title Note: The query returns titles, but the ordering is according to calories, which do not appear in the output. 26

27 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 every is true. 27

28 Quantifiers (Example) Recipes that have some ingredient with calories more than 300. for $r in doc("recipes.xml")//recipe where some $i in $r/ingredient satisfies gt 300 return $r/title Recipes where every ingredient has calories less than 300. for $r in doc("recipes.xml")//recipe where every $i in $r/ingredient satisfies lt 300 return $r/title 28

29 Element Fusion (1) To every recipe, add the attribute calories! <result> {let $rs := doc("recipes.xml")//recipe for $r in $rs return <recipe> {$r/title} </recipe>} an element an attribute </result> 29

30 Element Fusion (2) 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> 30

31 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 $rs := doc("recipes.xml")//recipe return distinct-values($rs//ingredient/@name) yields beef cube steak onion, sliced into thin rings... 31

32 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> 32

33 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> 33

34 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" 34

35 Running Queries with Galax Galax is an open-source implementation of XQuery ( The main developers have taken part in the definition of XQuery 35

36 Outline XQuery RDQL Querying on RDF Data SparQL 36

37 Introduction RDF Data Query Language JDBC/ODBC friendly Simple: SELECT FROM WHERE AND USING some information somewhere this match these constraints these vocabularies 37

38 Example 38

39 SELECT, FROM, WHERE The SELECT portion indicates which RDQL variables should be returned by the query. Variables are shown in form of?var The FROM part indicates the RDF sources to be queried. Each source is enclosed by angle brackets (< >). More than one sources are separated using commas. The WHERE part indicates the constraints that RDF triples must accomplish in order to be returned. 39

40 Example The query: SELECT?x FROM <doc.rdf> WHERE (?x, < "John Smith") returns: x ============================= < 40

41 Example Return all the resources that have property FN and the associated values: SELECT?x,?fname FROM <doc.rdf> WHERE (?x, < The outcome is: x fname ================================================ < "John Smith" < "Sarah Jones" < "Matt Jones" 41

42 Example Return the given name of Jones (which is a family): SELECT?givenName FROM <doc.rdf> WHERE (?y, < "Jones") (?y, < The outcome is: givenname ========= "Matthew" "Sarah" 42

43 URI Prefixes : USING RDQL has a syntactic convenience that allows prefix strings to be defined in the USING clause. Syntax: prefix FOR namespace 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 < 43

44 Filters Using AND part, the constraints on RDQL variables can be defined. SELECT?resource WHERE (?resource, info:age,?age) AND?age >= 25,?age < 50 USING info FOR < 44

45 Another Example SELECT?title?description?orbit?satellite?sensor?date FROM < 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 < dc FOR < rdf FOR < rdfs FOR < 45

46 Implementations Jena Sesame RDFStore 46

47 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 47

48 Outline XQuery RDQL SparQL Another RDF query language 48

49 Introduction An RDF query language developed by W3C. Builds on previous RDF query languages such as rdfdb, RDQL, and SeRQL. 49

50 Example RDF 50

51 Example (1) Simple Query: PREFIX foaf: < SELECT?url FROM <bloggers.rdf> WHERE {?contributor foaf:name "Jon Foobar".?contributor foaf:weblog?url. } 51

52 Term Constraints Restricting the Values of Strings: PREFIX foaf: < SELECT?name?depiction WHERE {?person foaf:name?name. FILTER regex (?name, Ali ) } 52

53 Term Constraints Restricting Numeric Values: PREFIX foaf: < SELECT?name?depiction WHERE {?person foaf:name?age. FILTER (?age > 50 ) } 53

54 Optional Values Optional block: SPARQL has a the ability to query for data but not to fail query when that data does not exist. E.g.,?mbox in the following example. PREFIX foaf: < SELECT?name?mbox WHERE {?person foaf:name?name. OPTIONAL {?person foaf:mbox?mbox. } } 54

55 Example (3) Alternative matches: one of a number of possibilities is tried. 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. Refer to references for more information. 55

56 References A Programmer's Introduction to RDQL

57 Any Question... 57