Dr. Alexandra I. Cristea CS 253: Topics in Database Systems: XQuery

2 Previously we looked at: –XPath –Namespaces Next: –XQuery

3 Xquery

4 What is XQuery? XQuery is the language for querying XML data XQuery for XML is like SQL for databases XQuery is built on XPath expressions XQuery is defined by the W3C XQuery is supported by all the major database engines (IBM, Oracle, Microsoft, etc.) XQuery is a W3C recommendation (Jan 2007; latest 14 Dec 2010) thus a standard

5 XQuery - Examples of Use Extract information to use in a Web Service Generate summary reports Transform XML data to XHTML Search Web documents for relevant information

6 XQuery compared to XPath XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators. XQuery 1.0 is a strict superset of XPath 2.0 XPath 2.0 expression is directly an XQuery 1.0 expression (a query) The extra expressive power is the ability to: –Join information from different sources and –Generate new XML fragments

7 Xquery compilers Download: Or try out at*: Syntax check at:

8 XQuery query makeup Prolog –Like XPath, XQuery expressions are evaluated relatively to a context –explicitly provided by a prolog (header) ~ header with definitions Body –The actual query Generate Join Select

9 XQuery Ex.: Prolog + Query

10 XQuery Prolog (i.e., header(s)) Settings define various parameters for the XQuery processor language, such as: xquery version "1.0"; declare base-uri " declare default element namespace " declare namespace xs= " import module " at "logo.xq"; declare variable $x as xs:integer := 7; declare function addLogo($root as node()) as node()*{ }; (: etc :)

Module definition xquery version 1.0; module namespace mylib = declare variable $mylib:foo as xs:string := foo; declare function mylib:foobar() as xs:string { concat ($mylib:foo, bar) }; 11

12 Body: Constructors Direct constructors in Xquery: my fragment –Evaluates to the given XML fragment

13 Explicit constructors computed constructors

14 Variable bindings (implicit constructors) {$name} {$job} {$deptno} {$SGMLspecialist }

15 How to Select Nodes with XQuery? Functions –XQuery uses functions to extract data from XML documents. (X)Path Expressions –XQuery uses path expressions to navigate through elements in an XML document. Predicates –XQuery uses predicates to limit the extracted data from XML documents.

16 Functions doc() –function to open a file Example: –doc("books.xml") Note: A call to a function can appear where an expression may appear.

17 Path Expressions Example: select all the title elements in the "books.xml" file: doc("books.xml")/bookstore/book/title

18 Predicates Example: select all the book elements under the bookstore element that have a price element with a value that is less than 30 : doc("books.xml")/bookstore/book[price<30]

19 At a glance: function, path, predicate

20 FLWOR For, Let, Where, Order by, Return = main engine ~ SQL syntax (SFW(GH)O) ~ programs and function calls

21 FLWOR by comparison with Path expressions select all the title elements under the book elements that are under the bookstore element that have a price element with a value that is higher than 30. Path expression : doc("books.xml")/bookstore/book[price>30]/title FLWOR expression : for $x in doc("books.xml")/bookstore/book where $x/price>30 return $x/title

22 Sorting in FLWOR for $x in doc("books.xml")/bookstore/book where $x/price>30 order by $x/title return $x/title

23 Present the Result In an HTML List { for $x in doc("books.xml")/bookstore/book/title order by $x return {$x} }

24 Result HTML List Everyday Italian Harry Potter Learning XML XQuery Kick Start

25 Eliminate element (here: title) { for $x in doc("books.xml")/bookstore/book/title order by $x return {data($x)} (: also text{} :) }

26 New result HTML List Everyday Italian Harry Potter Learning XML XQuery Kick Start

27 Another FLWOR Expression { for $s in doc("students.xml")//student let $m := $s/major where count($m) ge 2 order by return { $s/name/text()} }

28 The Difference between for and let

29 The Difference between for and let := in

30 The Difference between for and let

31 The Difference between for and let

32 FLWOR Basic Building Blocks

33 General rules for and let may be used many times in any order only one where is allowed many different sorting criteria can be specified (descending, ascending, etc.)

34 Reversing order Reverses the order of a sequence, for nodes or atomic values reverse (( 1, 2, 3)) -> 321

35 Joining documents for $p in doc(" for $n in doc("neighbors.xml")//neighbor[ssn = $p/ssn] return { $p/ssn } { $n/name } { $p/income }

36 Two-way join in a where Clause for $item in doc(ord.xml)//item, $product in doc(cat.xml)//product where = $product/number return <item name={$product/name} />

37 Aggregating Make summary calculations on grouped data Functions: –sum, avg, max, min, count

38 Conditionals for $b in doc(bib.xml)/book return {$b/title} {if ( count($b/author) and others ) }

39 Nesting Conditional Expressions Conditional expressions can be nested else if functionality is provided if ( count($b/author) = 1 ) then $b/author else if (count($b/author) = 2 )then (:.. :) else ( $b/author[1], and others )

40 Logical Expressions and, or operators: –and has precedence over or –Parentheses can change precedence if ($isDiscounted and ($discount > 5 or $discount < 0 ) ) then 5 else $discount not function for negations: if (not($isDiscounted)) then 0 else $discount

41 XQuery Built-in Functions XQuery function namespace URI is: default prefix: fn:. E.g.: fn:string() or fn:concat(). fn: is the default prefix of the namespace, the function names does not need to be prefixed when called.

42 Built-in Functions String-related –substring, contains, matches, concat, normalize- space, tokenize Date-related –current-date, month-from-date, adjust-time-to- timezone Number-related –round, avg, sum, ceiling Sequence-related –index-of, insert-before, reverse, subsequence, distinct-values

43 Built-in Functions (2) Node-related –data, empty, exists, id, idref Name-related –local-name, in-scope-prefixes, QName, resolve- QName Error handling and trapping –error, trace, exactly-one Document and URI-related –collection, doc, root, base-uri

44 Function calls doc("books.xml")//book[substring(title,1,5)='Harry'] let $name := (substring($booktitle,1,4)) {upper-case($booktitle)}

45 for $x in doc(" stea/courses/CS253/2009/books.xml")// book/title for $y in data($x) for $name in (substring($y,1,4)) return $name

46 User Defined Functions declare function prefix:function_name($parameter AS datatype) AS returnDatatype { (:...function code here... :) };

47 User-defined Functions declare function depth($e AS xsd:integer) AS xsd:integer { if (empty($e/*) then 1 else max(for $c in $e/* return depth($c)) ) +1 }; (: usage :) for $b in doc(bib.xml)/book return depth($b)

48 Existential and Universal Quantifiers for $b in doc(bib.xml)/book where some $author in $b/author satisfies $author/text() = Ullman return $b for $b in doc(bib.xml)/book where every $author in $b/author satisfies $author/text() = Ullman return $b Return books where all authors are Ullman Return books where at least one author is Ullman

49 Comments

50 Comparisons Value comparisons Eq, ne, lt, le, gt, ge Used to compare individual values Each operand must be a single atomic value (or a node containing a single atomic value) General comparisons =, !=,, >= Can be used with sequences of multiple items

51 Example

52 XQuery Syntax Declarative, functional language ~ SQL Nested expressions Case sensitive White spaces: –Tabs, space, CR, LF –Ignored between language constructs –Significant in quoted strings No special EOL character

53 Keywords and names Keywords and operators –Case-sensitive, generally lower case –May have several meanings depending on the context E.g. * or in –No reserved words All names must be valid XML names –variables, functions, elements, attributes –Can be associated with a namespace

54 XQuery gives you a choice: Path Expressions: –If you just want to copy certain elements and attributes as is FLWOR Expressions: –Allow sorting –Allow adding elements/attributes –Verbose, but can be clearer

55 XQuery tools XStylus Studio oad.html (free trial version) oad.html –See also short XQuery intro at:

56 Other info: –XQuery on Distributed Resources –Extensions for generic programming with XML

57 XQuery on Distributed Sources

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63 XML and programming XSLT, XPath and XQuery provide tools for specialized tasks. But many applications are not covered: –domain-specific tools for concrete XML languages –general tools that nobody has thought of yet

64 XML in general-purpose programming languages parse XML documents into XML trees navigate through XML trees construct XML trees output XML trees as XML documents DOM and SAX are corresponding APIs that are language independent and supported by numerous languages. JDOM is an API that is tailored to Java.DOMSAXJDOM

65 XQuery Conclusion We have learned: –XQuery definition –Usage scenarios –Comparison w. XSLT and XPath –Capabilities –Functions, path expressions and predicates –FLWOR