3. 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. 4
An Address Book as
an XML document
<addresses>
<person>
<name> Donald Duck</name>
<tel> 414-222-1234 </tel>
<email> donald@yahoo.com </email>
</person>
<person>
<name> Miki Mouse</name>
<tel> 123-456-7890 </tel>
<email>miki@yahoo.com</email>
</person>
</addresses>
5. 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. 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. 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. 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. 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. 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>
12. 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. 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. 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., Œ for
the Hebrew letter Mem
15. 15
XML Structure
• Nesting tags can be used to express
various structures, e.g., a tuple
(record):
<person>
<name> Lisa Simpson</name>
<tel> 02-828-1234 </tel>
<tel> 054-470-777 </tel>
<email> lisa@cs.huji.ac.il </email>
</person>
16. 16
XML Structure (cont’d)
• We can represent a list by using the
same tag repeatedly:
<addresses>
<person> … </person>
<person> … </person>
<person> … </person>
<person> … </person>
…
</addresses>
18. 18
Terminology
The segment of an XML document
between an opening and a corresponding
closing tag is called an element
<person>
<name> Bart Simpson </name>
<tel> 02 – 444 7777 </tel>
<tel> 051 – 011 022 </tel>
<email> bart@tau.ac.il </email>
</person>
element element,
a sub-element of
not an element
19. 19
An XML Document is a Tree
person
name emailtel tel
Bart Simpson
02 – 444 7777
051 – 011 022
bart@tau.ac.il
Leaves are either empty or contain PCDATA
20. 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. 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
23. 23
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>
We want to see
the text as is,
even though
it includes tags
24. 24
A Complete XML Document
http://www.mscs.mu.edu/~praveen/Teaching/fa
25. 25
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
An XML document
must be well formed
26. 26
Representing relational
databases
A relational database for school:
student: course:
enroll:
cno title credit
331 DB 3.0
350 Web 3.0
… … …
id name gpa
001 Joe 3.0
002 Mary 4.0
… … …
id cno
001 331
001 350
002 331
… …
27. 27
XML representation
<school>
<student id=“001”>
<name> Joe </name> <gpa> 3.0 </gpa>
</student>
<student id=“002”>
<name> Mary </name> <gpa> 4.0 </gpa>
</student>
<course cno=“331”>
<title> DB </title> <credit> 3.0 </credit>
</course>
<course cno=“350”>
<title> Web </title> <credit> 3.0 </credit>
</course>
