XML and Web Data - cs.sfu.ca · XML and Web Data. CMPT 354: Database I -- XML 2 Data in HTML • HyperText Markup Language – Different data elements are set out using tags • No

XML and Web Data

CMPT 354: Database I -- XML 2

Data in HTML

• HyperText Markup Language– Different data elements

are set out using tags• No schema?

– Based on the data itself, we can make a reasonable guess about the structure

– “Self-describing”


Object and Schema


Semi-structured Data

• Object-like: it can be represented as a collection of objects

• Schemaless: it is not guaranteed to conform to any type structure

• Self-describing– Often carries only the names of the attributes

and has a lower degree of organization than the data in the database

• Semi-structured data: data with the above characteristics


Schemaless But Self-Describing

(#12345,[ListName:“Students”,Contents:{ [Name:“John Doe”,

ID:“111111111”,Address:[Number:123, Street:“Main St”] ],[Name:“Joe Public”,Id:“666666666”,Address:[Number:666, Street:“Hollow Rd”] ]}

] )


XML• Extensible Markup Language

– A standard adopted in 1998 by the W3C (World Wide Web Consortium)

• Optional mechanisms for specifying document structure– DTD: the Document Type Definition Language, part of

the XML standard– XML Schema: a more recent specification built on top of

XML• Query languages for XML

– XPath: lightweight– XSLIT: document transformation language– XQuery: a full-blown language


From HTML to XML


HTML and XML

• HTML– A fixed number of tags– Each tag has its own well-defined meaning

• E.g., <table> … </table>

• XML: HTML-like language– An arbitrary number of user-defined tags– No a priori semantics– Mainly for data exchange– Display using stylesheet


Important Differences

• XML contains a large assortment of tags chosen by the document author– The only valid tags in HTML are those sanctioned by

the official specification of the language; other tags are ignored

• Every opening tag must have a matching closing tag, and the tags must be properly nested – E.g., <a><b></a></b> is not allowed– Some HTML tags are not required to be closed, e.g.,

<p>• The document has a root element – the element

that contains all other elements


Example

Root element

Mandatory statement

XML elements

Element names

Element contents


Hierarchical StructurePersonList Student

Title Contents

Person Person

Name: John Doe

Id: 111111111

Address

Number: 123

Street: Main St

Name: Joe Public

Id: 666666666

Address

Number: 666

Street: Hollow Rd


Attributes

• <PersonList Type=“Student”>– Type is the name of an attribute that belongs to the

element PersonList– Student is the attribute value– All attribute values must be quoted– Text strings between tags do not need to be quoted

• Empty element– <Title Value=“Student List”/>– The element has one attribute and no content– A shorthand for <Title Value=“Student List”></Title>


Processing Instructions & Comments

• Processing instructions– <?xml version=“1.0” ?>– Contain anything the author might want to communicate

to the XML processor, e.g., <?my-command go bring coffee?>

– Rarely used• Comment

– – Can occur everywhere except inside the markups, i.e.,

between symbols < and >– An integral part of the document– May be used by a receiver (e.g., a browser)


CDATA Construct

• Include strings of characters which contain markup elements that might make the document ill formed

• <![CDATA[ This is an example of markup in HTML: <b><i> Example <\b><\i>]]>


XML Elements and Data Objects

• XML allows mixed data/text structure• XML elements are ordered• XML has only one primitive type, string, and

very weak facilities for specifying constraints<Address>

<Number> 123 </Number><Street> Main St </Street>

</Address>is different from<Address>

<Street> Main St </Street><Number> 123 </Number>

</Address>

A legal XML document<Address>

Sally lives on<Street> Main St </Street>house number<Number> 123 </Number>in the beautiful Anytown, Canada.

</Address>


Use of Attributes• An element can have any number of user-defined

attributes• What attributes can do can also be achieved with elements

– An attribute may occur only once within a tag, while subelementswith the same tag may be repeated

• Attributes introduce ambiguity as to whether to represent information as attributes or elements– Sometimes convenient for representing data, can also be done with

elements– The use of attributes is expected to decline

<Address><Number> 123 </Number><Street> Main St </Street>

</Address>

<Address Number=“123” Street=“Main St/>


Attributes in Markup<Act Number=“5”>

<Scene Number=“1” Place=“Mantua. A street”>…<Apothecary Voice=“scared”>

Such mortal drugs I have; but Mantua’s lawIs death to any he that utters them.

</Apothecary><Romeo Voice=“persistent”>

Art thou so bare and full of wretchedness,And fear’st to die?…

</Romeo>…

</Scene></Act>


Advantages of Attributes

• Attributes in an element are not ordered– <Address Number=“123” Street=“Main St”/>– <Address Street=“Main St” Number=“123”/>

• Attributes are more succinct• Attributes can be declared to have unique value

and can be used to enforce limited kind of referential integrity

<Address><Number> 123 </Number><Street> Main St </Street>

</Address>


ID and IDREF – Cross-References


Well Formed XML Document

• It has a root element• Every opening tag is followed by a matching

closing tag, and the elements are properly nested inside each other

• Any attribute can occur at most once in a given opening tag, its value must be provided, and this value must be quoted


Namespaces

• A term (tag) might have different meanings in different contexts– <name><First>John</First> <Last>Doe</Last></Name>– <Name>Simon Fraser University</Name>

• Every XML tag must have two parts: namespace and local name– General structure: namespace:local-name– Namespace represented by URI (uniform resource

identifier)• An abstract identifier (a general unique string)• URL (uniform resource locator)


Example – Namespace• Namespaces are defined using the attribute xmlns

– All names xml* should be considered reserved• Default namespace xmlns=“…”

– Only one default namespace• Other namespace xmlns:toy=“…”

– Prefixes (e.g., toy) must be distinct<item xmlns=“http://www.acmeinc.com/jp#supplies”

xmlns:toy=“http://www.acmeinc.com/jp#toys”><name>backpack</name><feature>

<toy:item><toy:name>cyberpet</toy:name>

</toy:item></feature>

</item>


Namespace Declarations

• Namespace as prefix– E.g., toy:item, toy:name– Tags without prefix belong to the default

namespace• Namespace declarations have scope

– Can be nested like a program block


Example – Scopes of Namespaces

<item xmlns=“http://www.acmeinc.com/jp#supplies”xmlns:toy=“http://www.acmeinc.com/jp#toys”>

<name>backpack</name><feature>

<toy:item><toy:name>cyberpet</toy:name>

</toy:item></feature><item xmlns=“http://www.acmeinc.com/jp#supplies2”

xmlns:toy=“http://www.acmeinc.com/jp#toys2”><name>notebook</name><toy:name>sticker</toy:name>

</item></item>


More About Namespace

• The name of a namespace is just a string that happens to be a URL

• Not necessarily it is a real address that contains some kind of schema describing the corresponding set of names

• Don’t be misled by the URL!


Summary

• HTML and XML: differences and applications

• Structure of XML– Elements– Attributes– Well formed XML documents

• Namespace


To-Do-List

• Can every relational table be represented in XML? Can every XML document be represented in a relational table?

• RSS is an application of XML. Try to understand the two RSS segments at http://www.xml.com/pub/a/2002/12/18/dive-into-xml.html

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XML and Web Data - cs.sfu.ca · XML and Web Data. CMPT 354: Database I -- XML 2 Data in HTML • HyperText Markup Language – Different data elements are set out using tags • No