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Topics in Database Systems: XQuery

Topics in Database Systems: XQuery. Jonathan Foss Dr. Alexandra I. Cristea http://www.dcs.warwick.ac.uk/~acristea/. Previously we looked at: XPath Namespaces Next: XQuery. XQuery http://www.w3.org/TR/xquery/. What is XQuery?. XQuery is the language for querying XML data

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Topics in Database Systems: XQuery

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  1. Topics in Database Systems: XQuery Jonathan Foss Dr. Alexandra I. Cristea http://www.dcs.warwick.ac.uk/~acristea/

  2. Previously we looked at: • XPath • Namespaces • Next: • XQuery

  3. XQueryhttp://www.w3.org/TR/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) 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’ • Free 30 day trial download at http://www.altova.com/download/xmlspy/xml_editor_enterprise.html • Syntax check at: http://www.w3.org/2007/01/applets/xqueryApplet.html • Other open source tools such as Xqilla • See http://www.w3.org/XML/Query/#implementations

  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"; module namespace math = "http://example.org/math-functions"; (: module declaration: resource in lib :) declare base-uri "http://example.org"; declare default element namespace "http://example.org/names"; declare namespace xs= "http://www.w3.org/2001/XMLSchema"; import module "http://www.w3.org/2003/05/xpath-functions" at "logo.xq"; declare variable $x as xs:integer := 7; declare function addLogo($root as node()) as node()*{ }; (: etc :)

  11. Body: ConstructorsDirect constructors in Xquery: <XMLfragment>my fragment </XMLfragment> • Evaluates to the given XML fragment

  12. Explicit constructors computed constructors

  13. Variable bindings (implicit constructors) <employee empid="{$id}"> <name>{$name}</name> {$job} <deptno>{$deptno}</deptno> <salary>{$SGMLspecialist+100000}</salary> </employee>

  14. 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.

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

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

  17. 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]

  18. At a glance: function, path, predicate

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

  20. 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

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

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

  23. Result HTML List <ul> <li><title lang="en">Everyday Italian</title></li> <li><title lang="en">Harry Potter</title></li> <li><title lang="en">Learning XML</title></li> <li><title lang="en">XQuery Kick Start</title></li> </ul>

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

  25. New result HTML List <ul> <li>Everyday Italian</li> <li>Harry Potter</li> <li>Learning XML</li> <li>XQuery Kick Start</li> </ul>

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

  27. The Difference between for and let

  28. The Difference between for and let := in

  29. The Difference between for and let

  30. The Difference between for and let

  31. FLWOR Basic Building Blocks

  32. 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.)

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

  34. Joining documents for $p in doc("www.irs.gov/taxpayers.xml")//person for $n in doc("neighbors.xml")//neighbor[ssn = $p/ssn] return <person> <ssn> { $p/ssn } </ssn> { $n/name } <income> { $p/income } </income> </person>

  35. Two-way join in a where Clause for $item in doc(“ord.xml”)//item, $product in doc(“cat.xml”)//product where $item/@num = $product/number return <item num=“{$item/@num}” name=“{$product/name}” quan=“{$item/@quantity}” />

  36. Aggregating • Make summary calculations on grouped data • Functions: • sum, avg, max, min, count

  37. Conditionals for $b in doc(“bib.xml”)/bookreturn  <short>   {$b/title}   <author>    {if (count($b/author) < 3 )then   $b/authorelse        ( $b/author[1], <author>and others</author>)      }    </author>  </short>

  38. Nesting Conditional Expressions • Conditional expressions can be nested • ‘else if’ functionality is provided • if ( count($b/author) = 1 )then   $b/authorelse if (count($b/author) = 2 )then(: .. :)        else ( $b/author[1], <author>and others</author>)

  39. 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

  40. XQuery Built-in Functions XQuery function namespace URI is:http://www.w3.org/2005/02/xpath-functions 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.

  41. 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

  42. 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

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

  44. for $x in doc("http://www.dcs.warwick.ac.uk/~acristea/courses/CS253/2009/books.xml")//book/title for $y in data($x) for $name in (substring($y,1,4)) return $name

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

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

  47. Existential and Universal Quantifiers • for $b in doc(“bib.xml”)/bookwhere some $author in $b/authorsatisfies $author/text() = “Ullman”return $b • for $b in doc(“bib.xml”)/bookwhere every $author in $b/authorsatisfies $author/text() = “Ullman”return $b Return books where at least one author is “Ullman” Return books where all authors are “Ullman”

  48. Comments

  49. 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

  50. Example

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