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

transcript

Dr. Alexandra I. Cristea

http://www.dcs.warwick.ac.uk/~acristea/

CS 253: Topics in Database Systems: XQuery

• Previously we looked at:– XPath– Namespaces

• Next:– XQuery

Xqueryhttp://www.w3.org/TR/xquery/

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

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

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

Xquery ‘compilers’

•Download: http://www.altova.com/altovaxml.html •Or try out at*:•http://support.x-hive.com/xquery/index.html•Syntax check at: http://www.w3.org/2007/01/applets/xqueryApplet.html

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

XQuery Ex.: Prolog + Query

XQuery Prolog (i.e., header(s))• Settings define various parameters for the XQuery processor language,

such as:xquery version "1.0";

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 :)

Module definition

xquery version “1.0”;module namespace mylib = “http://www.example.com/test_library”;

declare variable $mylib:foo as xs:string := “foo”;

declare function mylib:foobar() as xs:string

concat ($mylib:foo, “bar”)

Body: Constructors

Direct constructors in Xquery:

<XMLfragment>my fragment </XMLfragment>

– Evaluates to the given XML fragment

Explicit constructors

computed constructors

Variable bindings (implicit constructors)

{$job} <deptno>{$deptno}</deptno> <salary>{$SGMLspecialist+100000}</salary>

</employee>

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.

Functions

• doc() – function to open a file

• Example:– doc("books.xml")

• Note: A call to a function can appear where an expression may appear.

Path Expressions

• Example:select all the title elements in the "books.xml"

doc("books.xml")/bookstore/book/title

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]

At a glance: function, path, predicate

• For, Let, Where, Order by, Return

= main engine

~ SQL syntax (SFW(GH)O)

~ programs and function calls

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

Sorting in FLWOR• for $x in doc("books.xml")/bookstore/book

where $x/price>30

order by $x/title

return $x/title

Present the Result In an HTML List

for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>{$x}</li>

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>

Eliminate element (here: title)

for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>{data($x)}</li> (: also text{} :)

New result HTML List

<li>Everyday Italian</li>

<li>Harry Potter</li>

<li>Learning XML</li>

<li>XQuery Kick Start</li>

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>

The Difference between for and let

FLWOR Basic Building Blocks

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.)

Reversing order

• Reverses the order of a sequence, for nodes or atomic values

• reverse (( 1, 2, 3))

-> 321

Joining documentsfor $p in doc("www.irs.gov/taxpayers.xml")//person

for $n in doc("neighbors.xml")//neighbor[ssn = $p/ssn]

return

{ $n/name }

<income> { $p/income } </income>

</person>

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}” />

Aggregating

• Make summary calculations on grouped data

• Functions:– sum, avg, max, min, count

Conditionalsfor $b in doc(“bib.xml”)/book

return <short> {$b/title} <author> {if ( count($b/author) < 3 ) then $b/author else ( $b/author[1], <author>and others</author>) } </author> </short>

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], <author>and others</author>)

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

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.

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

Built-in Functions (2)• Node-related

– data, empty, exists, id, idref

• Name-related– local-name, in-scope-prefixes, QName, resolve-

• Error handling and trapping– error, trace, exactly-one

• Document and URI-related– collection, doc, root, base-uri

Function calls

doc("books.xml")//book[substring(title,1,5)='Harry']

let $name := (substring($booktitle,1,4))

<name>{upper-case($booktitle)}</name>

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

User Defined Functions

declare function prefix:function_name($parameter AS datatype)

AS returnDatatype

{ (: ...function code here... :) };

User-defined Functionsdeclare 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)

Existential and Universal Quantifiers

• for $b in doc(“bib.xml”)/bookwhere some $author in $b/author satisfies $author/text() = “Ullman”return $b

• for $b in doc(“bib.xml”)/bookwhere 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”

Comments

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

Example

XQuery Syntax• Declarative, functional language

• Nested expressions• Case sensitive• White spaces:

– Tabs, space, CR, LF– Ignored between language constructs– Significant in quoted strings

• No special EOL character

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

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

XQuery tools

• XStylus Studio 2007 http://www.stylusstudio.com/xml_download.html (free trial version)– See also short XQuery intro at:

http://www.stylusstudio.com/xquery_primer.html

Other info:

–XQuery on Distributed Resources

–Extensions for generic programming with XML

XQuery on Distributed Sources

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

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.

XQuery Conclusion

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

predicates– FLWOR

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

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