CS 253: Topics in Database
Systems: XQuery
Dr. Alexandra I. Cristea
http://www.dcs.warwick.ac.uk/~acristea/
• Previously we looked at:
– XPath
– Namespaces
• Next:
– XQuery
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Xquery
http://www.w3.org/TR/xquery/
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What is XQuery?
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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
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XQuery - Examples of Use
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Extract information to use in a Web Service
Generate summary reports
Transform XML data to XHTML
Search Web documents for relevant
information
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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
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Xquery ‘compilers’
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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
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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
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XQuery Ex.: Prolog + Query
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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 :)
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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”)
};
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Body: Constructors
Direct constructors in Xquery:
<XMLfragment>my fragment </XMLfragment>
– Evaluates to the given XML fragment
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Explicit constructors
computed constructors
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Variable bindings
(implicit constructors)
<employee empid="{$id}">
<name>{$name}</name>
{$job} <deptno>{$deptno}</deptno>
<salary>{$SGMLspecialist+100000}</salary>
</employee>
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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.
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Functions
• doc()
– function to open a file
• Example:
– doc("books.xml")
• Note: A call to a function can appear
where an expression may appear.
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Path Expressions
• Example:
select all the title elements in the "books.xml"
file:
doc("books.xml")/bookstore/book/title
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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]
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At a glance: function, path, predicate
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FLWOR
• For, Let, Where, Order by, Return
= main engine
~ SQL syntax (SFW(GH)O)
~ programs and function calls
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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
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Sorting in FLWOR
• for $x in
doc("books.xml")/bookstore/book
where $x/price>30
order by $x/title
return $x/title
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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>
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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>
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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>
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New result HTML List
<ul>
<li>Everyday Italian</li>
<li>Harry Potter</li>
<li>Learning XML</li>
<li>XQuery Kick Start</li>
</ul>
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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>
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The Difference between for and let
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The Difference between for and let
:=
in
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The Difference between for and let
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The Difference between for and let
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FLWOR Basic Building Blocks
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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.)
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Reversing order
• Reverses the order of a sequence, for
nodes or atomic values
• reverse (( 1, 2, 3))
-> 321
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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>
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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}” />
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Aggregating
• Make summary calculations on grouped
data
• Functions:
– sum, avg, max, min, count
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Conditionals
for $b in doc(“bib.xml”)/book
return
<short>
{$b/title}
<author>
{if ( count($b/author) < 3 )
then $b/author
else
( $b/author[1], <author>and others</author>)
}
</author>
</short>
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Nesting Conditional Expressions
• Conditional expressions can be nested
• ‘else if’ functionality is provided
• if ( count($b/author) = 1 )
then $b/author
else if (count($b/author) = 2 )then (: ..
:)
else ( $b/author[1], <author>and
others</author>)
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Logical Expressions
• and, or operators:
– and has precedence over or
– Parentheses can change precedence
if ($isDiscounted and ($discount
> 5 or $discount < 0 ) ) then 5
else $discount
• not function for negations:
if (not($isDiscounted)) then 0
else $discount
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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.
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Built-in Functions
• String-related
– substring, contains, matches, concat, normalizespace, tokenize
• Date-related
– current-date, month-from-date, adjust-time-totimezone
• Number-related
– round, avg, sum, ceiling
• Sequence-related
– index-of, insert-before, reverse, subsequence,
distinct-values
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Built-in Functions (2)
• Node-related
– data, empty, exists, id, idref
• Name-related
– local-name, in-scope-prefixes, QName, resolveQName
• Error handling and trapping
– error, trace, exactly-one
• Document and URI-related
– collection, doc, root, base-uri
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Function calls
doc("books.xml")//book[substring(title,1,5)='Harry']
let $name := (substring($booktitle,1,4))
<name>{upper-case($booktitle)}</name>
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for $x in
doc("http://www.dcs.warwick.ac.uk/~acri
stea/courses/CS253/2009/books.xml")//
book/title
for $y in data($x)
for $name in (substring($y,1,4))
return $name
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User Defined Functions
declare function
prefix:function_name($parameter AS datatype)
AS returnDatatype
{
(: ...function code here... :) };
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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)
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Existential and Universal Quantifiers
• for $b in doc(“bib.xml”)/book
where some $author in $b/author
satisfies $author/text() = “Ullman”
return $b
Return books where at least
one author is “Ullman”
• 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”
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Comments
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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
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Example
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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
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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
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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
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XQuery tools
• XStylus Studio 2007
http://www.stylusstudio.com/xml_downl
oad.html (free trial version)
– See also short XQuery intro at:
http://www.stylusstudio.com/xquery_primer
.html
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Other info:
–XQuery on Distributed Resources
–Extensions for generic programming with XML
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XQuery on Distributed Sources
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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
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XML in general-purpose
programming languages
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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.
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XQuery Conclusion
• We have learned:
– XQuery definition
– Usage scenarios
– Comparison w. XSLT and XPath
– Capabilities
– Functions, path expressions and
predicates
– FLWOR
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