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XML Technologies and Applications
Rajshekhar Sunderraman
Department of Computer Science
Georgia State University
Atlanta, GA 30302
raj@cs.gsu.edu
I: Introduction and XML Basics
December 2005December 2005

Outline
 Introduction
 XML Basics
 XML Structural Constraint Specification
 Document Type Definitions (DTDs)
 XML Schema
 XML/Database Mappings
 XML Parsing APIs
 Simple API for XML (SAX)
 Document Object Model (DOM)
 XML Querying and Transformation
 XPath
 XQuery
 XSLT
 XML Applications

Introduction
 XML: A W3C standard to complement HTML
 Two facets of XML: document-centric and data-centric
 Motivation
 HTML describes presentation
 XML describes content
 User defined tags to markup “content”
 Text based format.
 Ideal as “Data Interchange” format.
 Key technology for “distributed” applications.
 All major database products have been retrofitted with facilities to
store and construct XML documents.
 XML is closely related to object-oriented and so-called semi-
structured data.

Semistructured Data
An HTML document (student list) to be displayed on the Web
<dt>Name: John Doe
<dd>Id: s111111111
<dd>Address:
<ul>
<li>Number: 123</li>
<li>Street: Main</li>
</ul>
</dt>
<dt>Name: Joe Public
<dd>Id: s222222222
… … … …
</dt>
HTML does not distinguish
between attributes and values

Semistructured Data (cont’d.)
 To make the previous student list suitable for machine
consumption on the Web, it should have the following
characteristics
 Be object-like
 Be schemalessschemaless (not guaranteed to conform exactly to any
schema, but different objects have some commonality among
themselves.
 Be self-describingself-describing (some schema-like information, like attribute
names, is part of data itself)
 Data with these characteristics are referred to as
semistructured.

Semi-structured Data Model
 Set of label-value pairs.
{name: "Alan",
tel: 2157786,
email: “a@abc.com”
}
 The values themselves may be structures
{name: {first: “Alan”, last: “Black”},
tel: 2157786,
email: “a@abc.com”
}
name tel email
“Alan” 2157786 “a@abc.com”
name tel email
2157786 “a@abc.com”
lastfirst
“Alan” “Black”
Graph Model: Nodes represent
objects connected by labeled edges
to values

 Duplicate labels allowed
{name: "Alan", tel: 2157786, tel: 2498762"}
 The syntax is easily generalized to describe sets of objects
{person: {name: “Alan”,tel: 2157786,email: “a@abc.com”}
person: {name: “Sara”,tel: 2136877,email: “sara@abc.com”}
person: {name: “Fred”,tel: 7786312,email: “fred@abc.com”}
}
 All objects within a set need not have the same structure
{person:{name: “Alan”,tel: 2157786,email: “a@abc.com”},
person:{name: {first: “Sara”,last: “Black”},email: “s@abc.com”},
person:{name: “Fred”, tel: 7786312, height: 168}
}

 Relational Data is easily represented
{r1: {row: {a: a1, b: b1, c: c1},
{row: {a: a2, b: b2, c: c2}},
r2: {row: {c: c2, d: d2},
row: {c: c3, d: d3},
row: {c: c4, d: d4}}
}
 Object-oriented data is also naturally represented (each node has a
unique object id, either explicitly mentioned or system generated)
{person: &o1{name: “Mary”, age: 45,
child: &o2, child: &o3},
person: &o2{name: “John”, age: 17,
relatives: {mother: &o1, sister: &o3}},
person: &o3{name: “Jane”, country: “Canada”, mother: &o1}
}

 Formal syntax for semi-structured data model
<ssd-expr> ::== <value> | oid <value> | oid
<value> ::== atomicvalue | <complexvalue>
<complexvalue> ::==
{ label:<ssd-expr>, ..., label:<ssd-expr> }
 An oid value is said to be DEFINED if it appears before a value;
otherwise it is said to be USED
 An ssd-expression is CONSISTENT if
 An oid is defined at most once and
 If an oid is used, it must also be defined.
 A flexible and powerful data model that is capable of representing data
that does not have to follow the strict rules of databases.

What is Self-describing Data?
Non-self-describing (relational, object-oriented):
Data part:
(#12345, [“Students”, {[“John”, s111111111, [123,”Main St”]],
[“Joe”, s222222222, [321, “Pine St”]] }
] )
Schema part:
PersonListPersonList[ ListName: String,
Contents: [ Name: String,
Id: String,
Address: [Number: Integer, Street: String] ]
]

What is Self-Describing Data? (contd.)
 Self-describing:
 Attribute names embedded in the data itself, but are distinguished
from values
 Doesn’t need schema to figure out what is what (but schema might
be useful nonetheless)
(#12345,
[ListName: “Students”,
Contents: { [ Name: “John Doe”,
Id: “s111111111”,
Address: [Number: 123, Street: “Main St.”] ] ,
[Name: “Joe Public”,
Id: “s222222222”,
Address: [Number: 321, Street: “Pine St.”] ] }
] )

XML – The De Facto Standard for Semi-structured Data
 XML: eXXtensible MMarkup LLanguage
 Suitable for semi-structured data and has become a standard
 Used to describe content rather than presentation
 Differs from HTML in following ways
 New tags may be defined at will by the author of the document
(extensible)
 No semantics behind tags. For instance, HTML’s <table>…<table>…
</table></table> means: render contents as a table; in XML: doesn’t
mean anything special.
 Structures may be nested arbitrarily
 XML document may contain an optional schema that describes
its structure
 Intolerant to bugs; Browsers will render buggy HTML pages but
XML processors will reject ill-formed XML documents.

XML Syntax
XML Elements
element: piece of text bounded by user-defined matching tags:
<person>
<name>Alan</name>
<age>42</age>
<email>agb@abc.com</email>
</person>
Note:
 Element includes the start and end tag
 No quotation marks around strings; XML treats all data as text. This is
referred to as PCDATA (Parsed Character Data).
 Empty elements:
<married></married> can be abbreviated to <married/>

XML Syntax - Continued
Collections are expressed using repeated structures.
Ex. The collection of all persons on the 4th floor:
<table>
<description>People on the 4th floor</description>
<people>
<person>
<name>Alan</name><age>42</age<<email>agb@abc.com</email>
</person>
<person>
<name>Patsy</name><age>36</age><email>ptn@abc.com</email>
</person>
<person>
<name>Ryan</name><age>58</age><email>rgz@abc.com</email>
</person>
</people>
</table>

XML Attributes
• Attributes define some properties of elements
• Expressed as a name-value pairs
<product>
<name language="French">trompette six trous</name>
<price currency="Euro">420.12</price>
<address format="XLB56" language="French">
<street>31 rue Croix-Bosset</street>
<zip>92310</zip>
<city>Sevres</city>
<country>France</country>
</address>
</product>
• As with tags, user may define any number of attributes
• Attribute values must be enclosed within quotation marks.

Attributes vs Elements
 A given attribute can occur only once within a tag; Its value is always a
string
 On the other hand tags defining elements/sub-elements can repeat any
number of times and their values may be string data or sub-elements
 Same data may be encoded using attributes or elements or a combination
of the two
<person name="Alan" age="42">
</person>
or
<person name="Alan">
<age>42</age>
</person>

XML References
 Use id attribute to define a reference (similar to oids)
 Use idref attribute (in an empty element) to refer to a previously defined
reference.
<state id="s2"> -- defines an id or a reference
<scode>NE</scode>
<sname>Nevada</sname>
</state>
<city id="c2">
<ccode>CCN</ccode>
<cname>Carson City</cname>
<state-of idref="s2"/> -- refers to object called s2;
-- this is an empty element
</city>

Mixing Elements and Text
 XML allows us to mix PCDATA and sub-elements within an element.
<person>
This is my best friend
<name>Alan</name>
<age>42</age>
I am not sure of the following email
</person>
 This seems un-natural from a database perspective, but from a
document perspective, this is quite natural!

Order
• The semi-structured data model is based on unordered collections,
whereas XML is ordered. The following two pieces of semi-structured
data are equivalent:
person: {fname: "John", lname: "Smith:}
person: {lname: "Smith", fname: "John"}
but the following two XML data are not:
<person><fname>John</fname><lname>Smith</lname></person>
<person><lname>Smith></lname><fname>John</fname></person>
• To make matters worse, attributes are NOT ordered in XML; Following
two are equivalent:
<person fname="John" lname="Smith"/>
<person lname="Smith" fname="John"/>

Other XML Constructs
 Comments:

 Processing Instruction (PI):
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="classes.xsl"?>
Such instructions are passed on to applications that process XML
files.
 CDATA (Character Data): used to write escape blocks containing
text that otherwise would be considered markup:
<![CDATA[<start>this is not an element</start>]]>
 Entities: &lt stands for <

Well-Formed XML Documents
 An XML document is well-formed if
 Tags are syntactically correct
 Every tag has an end tag
 Tags are properly nested
 There is a root tag
 A start tag does not have two occurrences of the same
attribute
 An XML document must be well-formed before it can be
processed.
 A well-formed XML document will parse into a node-labeled tree

<?xml version=“1.0” ?>
<PersonList Type=“Student” Date=“2002-02-02” >
<Title Value=“Student List” />
<Person>
… … …
</Person>
<Person>
… … …
</Person>
</PersonList>
• Elements are nested
• Root element contains all others
Element (or tag)
names
Terminology
elements
RootRootelement
Empty
element
attributes

More Terminology
<Person Name = “John” Id = “s111111111”>
John is a nice fellow
<Address>
<Number>21</Number>
<Street>Main St.</Street>
</Address>
… … …
</Person>
Opening tag
Closing tag:
What is open must be closed
Nested element,
child of PersonPerson
Parent of AddressAddress,
Ancestor of numbernumber
“standalone” text, not
very useful as data,
non-uniform
Child of AddressAddress,
Descendant of PersonPerson
ContentofPersonPerson

XML Data Model
person
name emailtel tel
Bart Simpson
02–4447777
051–011022
bart@tau.ac.il
 Document Object Model (DOM) – DOM Tree
 Leaves are either empty or contain PCDATA
 Unlike ssd tree model, nodes are labeled with tags.

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