1. eXtensible Markup Language
XML
eXtensible Markup Language

2. Introduction and Motivation
Dr. Praveen Madiraju
Modified from Dr.Sagiv’s slides

3. XML vs. HTML HTML is a HyperText Markup language
Designed for a specific application, namely, presenting and linking hypertext documents
XML describes structure and content (“semantics”)
The presentation is defined separately from the structure and the content

4. An Address Book as an XML document
<addresses>
<person>
<name> Donald Duck</name>
<tel> </tel>
< > </ >
</person>
<name> Miki Mouse</name>
<tel> </tel>
</addresses>

5. Main Features of XML No fixed set of tags
New tags can be added for new applications
An agreed upon set of tags can be used in many applications
Namespaces facilitate uniform and coherent descriptions of data
For example, a namespace for address books determines whether to use <tel> or <phone>

6. Main Features of XML (cont’d)
XML has the concept of a schema
DTD and the more expressive XML Schema
XML is a data model
Similar to the semistructured data model
XML supports internationalization (Unicode) and platform independence (an XML file is just a character file)

7. XML is the Standard for Data Exchange
Web services (e.g., ecommerce) require exchanging data between various applications that run on different platforms
XML (augmented with namespaces) is the preferred syntax for data exchange on the Web

8. XML is not Alone
XML Schemas strengthen the data-modeling capabilities of XML (in comparison to XML with only DTDs)
XPath is a language for accessing parts of XML documents
XLink and XPointer support cross-references
XSLT is a language for transforming XML documents into other XML documents (including XHTML, for displaying XML files)
Limited styling of XML can be done with CSS alone
XQuery is a lanaguage for querying XML documents

9. The Two Facets of XML
Some XML files are just text documents with tags that denote their structure and include some metadata (e.g., an attribute that gives the name of the person who did the proofreading)
See an example on the next slide
XML is a subset of SGML (Standard Generalized Markup Language)
Other XML documents are similar to database files (e.g., an address book)

10. XML can Describe the Structure of a Document
<book year="1994">
<title>TCP/IP Illustrated</title>
<author>
<last>Stevens</last>
<first>W.</first>
</author>
<publisher>Addison-Wesley</publisher>
<price>65.95</price>
</book>

11. W3Schools Resources on XML Syntax

12. The Structure of XML XML consists of tags and text
Tags come in pairs <date> ... </date>
They must be properly nested
good
<date> ... <day> ... </day> ... </date>
bad
<date> ... <day> ... </date>... </day>
(You can’t do <i> ... <b> ... </i> ...</b> in HTML)

13. A Useful Abbreviation Abbreviating elements with empty contents:
<br/> for <br></br>
<hr width=“10”/> for <hr width=“10”></hr>
For example:
<family>
<person id = “lisa”>
<name> Lisa Simpson </name>
<mother idref = “marge”/>
<father idref = “homer”/>
</person>
...
</family>
Note that a tag may have a set of attributes, each consisting of a name and a value

14. XML Text XML has only one “basic” type – text
It is bounded by tags, e.g.,
<title> The Big Sleep </title>
<year> 1935 </ year> – 1935 is still text
XML text is called PCDATA
(for parsed character data)
It uses a 16-bit encoding, e.g., \&\#x0152 for the Hebrew letter Mem

15. XML Structure
Nesting tags can be used to express various structures, e.g., a tuple (record):
<person>
<name> Lisa Simpson</name>
<tel> </tel>
<tel> </tel>
< > </ >
</person>

16. XML Structure (cont’d)
We can represent a list by using the same tag repeatedly:
<addresses>
<person> … </person> …
</addresses>

17. XML Structure (cont’d)
<addresses>
<person>
<name> Donald Duck</name>
<tel> </tel>
< > </ >
</person>
<name> Miki Mouse</name>
<tel> </tel>
</addresses>

18. Terminology
The segment of an XML document between an opening and a corresponding closing tag is called an element
element
element,
a sub-element of
<person>
<name> Bart Simpson </name>
<tel> 02 – </tel>
<tel> 051 – </tel>
< > </ >
</person>
not an element

19. An XML Document is a Tree
person
name
tel
tel
Bart Simpson
051 –
02 –
Leaves are either empty or contain PCDATA

20. Mixed Content
An element may contain a mixture of sub-elements and PCDATA
<airline>
<name> British Airways </name>
<motto>
World’s <dubious> favorite</dubious>
airline
</motto>
</airline>

21. The Header Tag
<?xml version="1.0" standalone="yes/no" encoding="UTF-8"?>
Standalone=“no” means that there is an external DTD
You can leave out the encoding attribute and the processor will use the UTF-8 default

22. Processing Instructions
<?xml version="1.0"?>
<?xml-stylesheet  href="doc.xsl" type="text/xsl"?>
<!DOCTYPE doc SYSTEM "doc.dtd">
<doc>Hello, world!<!-- Comment 1 --></doc>
<?pi-without-data?>
<!-- Comment 2 -->
<!-- Comment 3 -->

23. Using CDATA We want to see the text as is, even though
it includes tags
Using CDATA
<HEAD1> Entering a Kennel Club Member
</HEAD1>
<DESCRIPTION> Enter the member by the name on his or her papers. Use the NAME tag. The NAME tag has two attributes. Common (all in lowercase, please!) is the dog's call name. Breed (also in all lowercase) is the dog's breed. Please see the breed reference guide for acceptable breeds. Your entry should look something like this:
</DESCRIPTION>
<EXAMPLE> <![CDATA[<NAME common="freddy" breed"=springer-spaniel">Sir Fredrick of Ledyard's End</NAME>]]>
</EXAMPLE>

24. Well-Formed XML Documents
An XML document (with or without a DTD) 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
Attribute values must be quoted.
An XML document must be well formed

25. Representing relational databases
A relational database for school:
student: course:
enroll:

26. XML representation <school> <student id=“001”>
<name> Joe </name> <gpa> </gpa>
</student>
<student id=“002”>
<name> Mary </name> <gpa> </gpa>
<course cno=“331”>
<title> DB </title> <credit> </credit>
</course>
<course cno=“350”>
<title> Web </title> <credit> </credit>

27. XML representation <enroll>
<id> 001 </id> <cno> </cno>
</enroll>
<id> 001 </id> <cno> </cno>
<id> 002 </id> <cno> </cno>
</school>