chapter 27 the world wide web and xml. copyright © 2004 pearson addison-wesley. all rights...

Chapter 27

The World Wide Web and XML

Copyright © 2004 Pearson Addison-Wesley. All rights reserved. 27-2

Topics in this Chapter

• The Web and the Internet• An Overview of XML• XML Data Definition• XML Data Manipulation• XML and Databases• SQL Facilities


The Web and the Internet

• Often thought of as synonymous, the Web and the Internet refer to two different arenas

• The Web is a gigantic amorphous database • The Internet is a giant network• URL’s are used to locate resources on the

network(Uniform Resource Locator/Identifier)

• Markup languages are used to interact with the database


Hypertext

• Hypertext Markup Language is a simple language for creating and displaying documents

• Hypertext Transfer Protocol(HTTP) is used to transfer these documents over the internet

• At each server data can be served up from system files, or from databases

• The databases on web servers can be SQL databases


XML

• XML provides extensions that permit the markup language to interact with hypertext as well as many other languages, including SQL, and so is useful when implementing web databases

• XML normally begins with a header called a declaration, followed by an element, consisting of start tag, character data, and end tag


XML

• XML normally begins with a header called a declaration, followed by an element, consisting of start tag, character data, and end tag

• XML declaration XML element start tag, character data, end tag

• <?xml version=”1.0”>• <greeting kind=“succinct”>Hello, World.</greeting>

greeting tag;kind=“succinct” XML attributeAttribute name is “kind”; value=“succinct”


XML History

• XML was created in 1996 to overcome limitations in SGML and HTML

• SGML is large and complicated• HTML fails to separate structural, semantic,

and formatting meta-data, and is not always “well-formed”

• XML has not supplanted HTML in web browsers, but is used in other areas, especially data interchange


XML History

SGML is large and complicated. It allow user to define their own tags and give their meaning

• <Paragraph> <Sentence>

<Subject> You</ Subject > <verb> Should specify</ verb > <Object>the <adjective> <emph1> first</ emph1 > </adjective> parameter </ Object ></ Sentence ><Sentence>……..</ Sentence >

• </Paragraph>


XML Applications

• Purchase orders, parts catalogues, and inventory records can be expressed in XML

• A database could consist of XML documents only, but it would NOT be relational

• XML can be used to represent relations, which could facilitate interchange between the internet and relational databases


XML Applications

• <?xml version+”1.0”>• <!– This is an XML representation of the parts relatoin --- >• <!-This partsdatabase Parts # PNAME COLOR WEIGHT CITY>

• <Partsrelation>

<Partstuple>

<PNUM> P1</PNUM>

<PNAME> NUT</ PNAME >

<COLOR> RED> </COLOR>

<WEIGHT> 12 </WEIGHT>

<CITY>LONDON </ CITY >

</Partstuple>

<Partstuple>

<PNUM> P2</PNUM>

<PNAME>left-wing-part-10th-part-Bolt</ PNAME >

<COLOR> Green </COLOR>


<CITY>Paris </ CITY >

</Partstuple>

• </Partsrelation>


XML Applications

• An XML information set is a document hierarchy


XML Hierarchy

• The root node is the top, and it has children• Each child has one parent• Relations are structured; XML documents are

said to be semi-structured, because its rules are looser

• An API to XML’s document object model supports retrieval, insertion, deletion and updates(pp901)


DTDs

• Document Type Definitions can be constructed using the DTD definition language

• DTDs are part of the XML standard• A DTD can mirror the structure of a relation

and then be used to format the output from queries

• In turn, the XML document produced can be used to generate a relation at the other end

• Text objects must be well-formed and valid


XML Applications

• <?xml version=”1.0”>• <!– This is an XML representation of the parts relatoin --- >• <DOCTYPE. . . .>

• <Partsrelation>

<Note>Revised Version </Note>

<Partstuple CITY= “LONDON” >

<PNUM> P1</PNUM>

<PNAME> NUT</ PNAME >


<Note>Part COLOR is Red by Default </Note>

</Partstuple>

<Partstuple COLOR=“GREEN”, CITY= “PARIS” >

<PNUM> P2</PNUM>

<PNAME>left-wing-part-10th-part-Bolt</ PNAME >

<COLOR> Green </COLOR>


</Partstuple>



XML Applications

• <?xml version+”1.0”>

• <!– This is an XML representation of the parts relation --- >• <!-Marker’s meaning ? Zero or one, * zero or more, +1 or more>• <DOCTYPE. . . .>• <Partsrelation>

• 1. <!elements Partsrelation (Note. Partuple*)><Note>Revised Version </Note> 2. <!elements Note(#PCDATA)>

<Partstuple CITY= “LONDON” >3. <!elements Partuple (PNUM. PNAME, WEIGHT, NOTE?)>

4. <attribute Partuple CITY(LONDON|Oslo|Paris) #required COLOR( Red|Green|Blue) “Red”>

<PNUM> P1</PNUM> 5. <!elementsPNUM (#PCDATA)>

<PNAME> NUT</ PNAME > 6. <!elementsPNAME (#PCDATA)>

<WEIGHT> 12 </WEIGHT> 7. <!elements WEIGHT (#PCDATA)>

<Note>Part COLOR is Red by Default </Note></Partstuple><Partstuple COLOR=“GREEN”, CITY= “PARIS” >

<PNUM> P2</PNUM> <PNAME>left-wing-part-10th-part-Bolt</ PNAME > <COLOR> Green </COLOR> <WEIGHT> 17 </WEIGHT></Partstuple>



Well-Formedness

• A textual object is well-formed if and only if:• It conforms to the grammar defined in the

XML standard• Any textual object it references is well-formed• Examples of fatal flaws: • Start and end tags don’t match, or are missing• More than one root element included


Validity

• A textual object is valid if and only if it is well-formed and it conforms to a specified DTD

• DTDs can support uniqueness and referential constraints via ID and IDREF attribute types

• These constraints do not function as keys, but can be used to transmit information from one relvar to another


Limitations of DTDs

• DTDs do not use XML syntax, and they cannot be processed by XML parsers

• Since everything in this arena is a character string, data type support is lacking

• They enforce an ordering of elements that is contra-relational

• They are still beneficial because they enforce a standard that is widely used


XML Schema

• XML Schema is an XML derivative, and can be interpreted by XML parsers

• Are written using a collection of names, from a name space (http://www.w3.org/2002/XMLSchema)

• The name space specification: xmlns:xsd=“http://www.w3.org/2002/XMLSchema”

• It is considerably more prolix• XML can enforce primitive types and some

derived types• XML types have essentially no operators

because “types” are still character strings

http://www.w3.org/2002/XMLSchema




XML Schema

• XML Schema is an XML derivative, and can be interpreted by XML parsers

• It is considerably more prolix• XML can enforce primitive types and some

derived types• XML types have essentially no operators

because “types” are still character strings


XML Data Manipulation

• XQuery is based on Xpath, which means that it is a read-only facility for traversing XMLs hierarchical paths

• Because XQuery can report horizontal and vertical subsets, and combine the results, it is said to support “select, project, and join”

• XUpdate is in the early planning stages, but presumably will support updates

• For now, only proprietary solutions to update


XML and Databases

• Three approaches:• Store XML documents as attributes• Shred documents into attributes• Store XML documents in “XML databases”


XML Documents as Attributes

• Define a new type, XMLDOC• As a new type, XMLDOC should have

operators defined, that can retrieve like XQuery, and that can check for well-formedness and validity


XML Documents Shred and Publish

• An XML document may be shredded into its components, which are then stored as attributes

• Attributes can be recombined and published as XML Documents

• This is an effective way for SQL databases to interact with the web

• Relational databases do not store hierarchies, nor are they intrinsically ordered, so shred and publish may not be “nonloss”


SQL Facilities

• XML Collection will offer support for shred and publish, where the publish feature supports publishing the XML data, and its schema

• XML Column will offer a new built-in type, XML that will come an XMLGEN operator to publish XML documents

• Database vendors offer built-in functions that can read and write elements within XML attribute values, e.g., XMLFILETOCLOB

chapter 27 the world wide web and xml. copyright © 2004 pearson addison-wesley. all rights...

Documents

character data

paris xml applications

web databasesxml

structured xml documents

server data

web servers

chapterthe web

web browsers