1. Dickson K.W. Chiu PhD, SMIEEE Thanks to Prof. Francis Lau (HKU)
CSIT600b: XML Programming
Xlink, XPointer, XQuery
Dickson K.W. Chiu
PhD, SMIEEE
Thanks to Prof. Francis Lau (HKU)

2. Problem of HTML Linking
Growth of the “Web” attributed to the ability of an HTML page to (hyper)link to another page
<a href= …
Limitations
Involves only two resources
One-way (the “back” button is a browser’s thing)
The source must be HTML
The whole document is loaded even if your link points to a section
<img src=“ …
The target page must contain explicit section markups
Dickson Chiu-2004

3. XML Linking
XLink to link resources, XPointer to point to fragments of XML documents
Tutorial:
W3C Recommendation 1.0, Jun 2001
Example
One-way and two-way
One link, involving 4 resources
Information about this link can be in a completely separated file
A.html
B.xml
C.html
D.xml
Dickson Chiu-2004

4. XLink Example XLink’s global attributes
<name xmlns=“ xmlns:xlink=“ <first>Tommy</first> <last xlink:type=“simple” xlink:href=“ </name>
XLink’s global attributes
type, href, label, role, arcrole, title, show, actuate, from, to
Can be added to elements that are in any arbitrary namespace
Dickson Chiu-2004

5. The “type” Attribute Mandatory, to define a link
Six types (the first 2 are link types, the others are for providing more info about a link)
Simple
Extended
Title – to provide human-readable titles
Locator – to indicate remote resources
Resource – to define local resources involved
Arc – to indicate the rules for traversing
Dickson Chiu-2004

6. “title”, “label”, and “role”
title and label are used to label the functions of various resources in the link
label is machine-readable, title human-readable
<last xlink:label=“lastName” xlink:type=“simple” xlink:href=“people.xml” xlink:title=“Last name” xlink:role=“ name </last>
role is a URI that references a resource that describes the meaning of the link
Dickson Chiu-2004

7. “actuate” and “show”
The actuate attribute specifies when the resource should be retrieved
onLoad (default)
onRequest
The show attribute specifies how to display the resource when it is loaded
new – new window
replace – current window
embed (default) – insert here like <html:img src=... >
The application can provide other values, using “QNames”, but the application should know how to handle them
xlink:actuate=“op:EveryFiveMinutes”
Dickson Chiu-2004

8. “from” and “to”
A link can involve multiple resources, and be traversed in multiple directions
<element xlink:from=“myapp:first” xlink:to=“myapp:last” xlink:show=“replace” xlink:actuate=“onRequest”> Click for last</element>
When clicked, the first link resource will be replaced by the last link resource
Dickson Chiu-2004

9. Simple Links Like HTML links: 2 resources
<element xlink:type=“simple” xlink:href=… xlink:show=“new”>Click me!</element>
Simple links are “inline” links because the link’s own content (i.e., “Click me”) is the resource
Dickson Chiu-2004

10. Extended Links
An element to describe links among sets of document(s), classified by labels
Extended links can be “out of line”
You could create a link from A to B even though you have no access rights for A and B
Locator-type to indicate remote resources, and arc-type to specify rules for traversals
Attribute arcrole is used to describe the relationship between the resources
Dickson Chiu-2004

11. Extended Links Example
<extendedlink xlink:type="extended">
<loc xlink:type="locator" xlink:href="..." xlink:label="parent" xlink:title="p1" />
<loc xlink:type="locator" xlink:href="..." xlink:label="parent" xlink:title="p2" />
<loc xlink:type="locator" xlink:href="..." xlink:label="child" xlink:title="c1" />
<loc xlink:type="locator" xlink:href="..." xlink:label="child" xlink:title="c2" />
<loc xlink:type="locator" xlink:href="..." xlink:label="child" xlink:title="c3" />
... <!-- arc-type elements would go here -->
</extendedlink>
<go xlink:type=“pararc" xlink:from="parent" xlink:to="child" />
links from parents to children (6 links total)
<go xlink:type=“childarc" xlink:to="child" />
links from parents to children and children to children (15 links total)
Upon clicking on a item, say “p1”, the rendering software (need not be a browser) displays a list of linked items for the user to choose.
Dickson Chiu-2004

12. XPointer Provides a method for pointing to pieces of an XML document
W3C Recommendation 1.0, Mar 2003
Built on XPath, with the following extensions
Locate information by string matching
Can be appended to URI’s
Address not only entire nodes, but pieces of nodes in an XML document
Finer than nodes
A location is any node type allowed in Xpath
A point is any spot in the XML document
A range is defined by two points
Dickson Chiu-2004

13. XInclude and XBase XML 1.0 / 1.1 does not have “include’s”
XInclude uses XPointer to include portions of another document
The parse attribute has value xml (parsed) or text (unparsed)
XBase is for providing base URI’s for relative links in a document
Dickson Chiu-2004

14. XQuery XQuery is a language for querying XML data
XQuery is built on XPath expressions
XQuery for XML is like SQL for databases
Updated release of the XQuery 1.0, XPath 2.0: W3C Oct 2004
XQuery is not an XML language - a version in XML syntax is called XQueryX.
Tutorial booklet download from DataDirect:
Brief tutorials:
Dickson Chiu-2004

15. XQuery W3C Requirements
at least one XML syntax (at least one human-readable syntax)
must be declarative
must be protocol independent
must respect XML data model
must be namespace aware
must coordinate with XML Schema
must work even if schemas are unavailable
must support simple and complex datatypes
must support universal and existential quantifiers
must support operations on hierarchy and sequence of document structures
must combine information from multiple documents
must support aggregation
must be able to transform and to create XML structures
must be able to traverse ID references
Dickson Chiu-2004

16. XQuery Basic Concepts Expressions are evaluated relative to a context:
namespaces
variables
functions
date and time
context item (current node or atomic value)
context position (in the sequence being processed)
context size (of the sequence being processed)
A query in XQuery is an expression that:
reads a sequence of XML fragments or atomic values
returns a sequence of XML fragments or atomic values
The principal forms of XQuery expressions are:
path expressions (same as XPath)
element constructors
FLWOR ("flower") expressions
list expressions
conditional expressions
quantified expressions
datatype expressions
Dickson Chiu-2004

17. XQuery Path Expressions
The simplest kind of query is just an XPath 2.0 expression.
Example:
document("recipes.xml")// recipe[title="Ricotta Pie"]//
Some XQuery specific extension of XPath:
location steps may follow a new IDREF axis
an arbitrary XQuery expression may be used as a location step
Dickson Chiu-2004

18. XPath 2.0 Major Changes from 1.0
now using XML Schema primitive types instead of just node-set, boolean, number, string
new type operators: cast, treat, assert, instance of
now using sequences instead of node-sets
also allow non-node types
new operators: for, if, some, every, intersect, except
many new functions
regular expression match/replace/tokenize
date formats
...
Dickson Chiu-2004

19. XQuery FLWOR expressions
The main engine of XQuery is FLWOR expression
For-Let-Where-Order-Return
pronounced "flower"
generalizes SELECT-FROM-HAVING-WHERE from SQL
Example
for $d in document("depts.xml")//deptno
let $e := document("emps.xml")//employee[deptno = $d]
where count($e) >= 10
order by avg($e/salary) descending
return
<big-dept> { $d,
<headcount>{count($e)}</headcount>, <avgsal>{avg($e/salary)}</avgsal>
}</big-dept>
For – loop
Let – binding once
Only 1 where
{ … } – list expression
Dickson Chiu-2004

20. XQuery FLWOR Join Example
Sufficient to compute joins of documents – example:
for $p IN document("
for $n IN document("neighbors.xml")//neighbor[ssn = $p/ssn]
return
<person>
<ssn> { $p/ssn } </ssn>
{ $n/name }
<income> { $p/income } </income>
</person>
Dickson Chiu-2004

21. XQuery List Expression
XQuery expressions manipulate lists of values, for which many operators are supported:
constant lists: (7, 9, <thirteen/>)
integer ranges: i to j
XPath expressions, like all named children of the context node: name
concatenation: ,
set operators: | (or union), intersect, except
functions: remove, index-of, count, avg, max, min, sum, distinct-values ...
When lists are viewed as sets:
XML nodes are compared on node identity
duplicates are removed
the order is preserved
Dickson Chiu-2004

22. XQuery Qualified Expression
some-in-satisfies
for $b in document("bib.xml")//book
where some $p in $b//paragraph satisfies
(contains($p,"sailing") AND contains($p,"windsurfing"))
return $b/title
every-in-satisfies
where every $p in $b//paragraph satisfies
contains($p,"sailing")
Dickson Chiu-2004

23. XQuery Datatype Expression
XQuery supports all datatypes from XML Schema, both primitive and complex types.
Constant values can be written:
as literals (like string, integer, float)
as constructor functions (true(), date(" "))
as explicit casts (cast as xsd:positiveInteger(47))
Arbitrary XML Schema documents can be imported into a query.
An instance of operator allows runtime validation of any value relative to a datatype or a schema.
A typeswitch operator allows branching based on types.
Dickson Chiu-2004

24. XQuery - Other Features
Hundreds of built-in operators and functions - contains anything you might think of
Computed element and attribute names - allow more flexible queries
User-defined functions - allow general-purpose computations
Views and updates are still under development
Dickson Chiu-2004

25. Status (updated) Software? Starting to come up
XLink 1.0 and Xbase June 27, 2001
XPointer March 2003
XInclude 1.0 – W3C Working Draft 10 Nov 2003
Updated release of the XQuery 1.0, XPath 2.0, Oct 2004
Working draft  Last call working draft  Candidate recommendation  Proposed recommendation  Recommendation
Dickson Chiu-2004