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The Web was originally built for human
consumption |
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Web pages are machine-readable but not machine-understandable |
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Example: A bibliography entry in HTML |
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<UL> |
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<LI> |
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R. Goldman, J. McHugh, and J. Widom. |
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<A
href="ftp://db.stanford.edu/pub/papers/xml.ps"> |
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From Semistructured Data to XML: Migrating the
Lore Data Model and Query Language |
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</A>. |
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Proceedings of the 2nd International Workshop on
the Web and Databases (WebDB ‘99), Philadelphia, Pennsylvania, June 1999. |
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</UL> |
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“XML is like HTML, where you make up your own
tags.” |
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Provides a uniform method for describing and
exchanging data using the HTTP protocol |
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HTML enables a universal method for displaying
data |
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Tag a word to be displayed in bold or italic |
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XML provides a universal method for describing
data |
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Declare data to be a retail price, a sales tax,
a book title, ... |
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Data is made up of characters or unparsed
“entities” |
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Subset of Standard Generalized Markup Language
(SGML) |
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Defined by the World Wide Web Consortium (W3C) |
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XML document |
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Optional prolog |
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One or more elements |
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Optional miscellany |
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Comments |
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Processing instructions |
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E.g., |
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<?xml version="1.0"?> |
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<greeting>Hello, world!</greeting> |
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<!-- A simple XML document. --> |
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Non-empty element |
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Start tag |
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Element name |
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Optional attribute specifications |
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Attribute name |
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Quoted string |
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Content |
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Elements |
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Character data |
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Entity or character references |
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Character data sections |
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Processing instructions |
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Comments |
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End tag |
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<Publication
URL="ftp://db.stanford.edu/pub/papers/xml.ps” |
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Authors="RG JM JW"> |
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<Title>From Semistructured Data ... Language</Title> |
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<Published>Proceedings of the ... Databases</Published> |
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<Location> |
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<City>Philadelphia</City> |
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<State>Pennsylvania</State> |
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</Location> |
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<Date> |
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<Month>June</Month> |
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<Year>1999</Year> |
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</Date> |
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</Publication> |
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<Author ID="RG">R.
Goldman</Author> |
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<Author ID="JM">J.
McHugh</Author> |
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<Author ID="JW">J.
Widom</Author> |
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<weather-report> |
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<date>March 25, 1998</date> |
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<time>08:00</time> |
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<area> |
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<city>Seattle</city> |
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<state>WA</state> |
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<region>West Coast</region> |
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<country>USA</country> |
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</area> |
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<measurements> |
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<skies>partly cloudy</skies> |
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<temperature>46</temperature> |
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... |
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</measurements> |
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</weather-report> |
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<p xml:lang="en">The quick brown
fox jumps over the lazy dog.</p> |
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<p xml:lang="en-GB">What colour
is it?</p> |
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<p xml:lang="en-US">What color
is it?</p> |
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<sp who="Faust" desc='leise'
xml:lang="de"> |
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<l>Habe nun, ach! Philosophie,</l> |
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<l>Juristerei, und Medizin</l> |
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<l>und leider auch Theologie</l> |
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<l>durchaus studiert mit heißem Bemüh'n.</l> |
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</sp> |
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Describes the syntax of a class of XML documents |
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Which elements are present |
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Structural relationships between the elements |
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Contains or points to markup declarations |
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Element type declarations |
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Attribute-list declarations |
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Entity declarations |
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Notation declarations |
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Examples |
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<!DOCTYPE greeting SYSTEM "http://www.
..."> |
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<!DOCTYPE greeting |
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[<!ELEMENT greeting (#PCDATA)> ]> |
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<!ELEMENT Name Content-Spec> |
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Content spec |
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EMPTY |
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ANY |
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Character data, optionally interspersed with
child elements |
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Child elements |
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Using a simple grammar governing the allowed
types of the child elements and the order in which they may appear |
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Examples |
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<!ELEMENT MEMO
(TO,FROM,SUBJECT,BODY,SIGN)> |
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<!ELEMENT BODY (P+)> |
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<!ATTLIST Name Attribute-Definition* > |
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Attribute definition |
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Name Attribute-Type Default-Declaration |
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Attribute type |
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String type |
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A set of tokenized types |
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Enumerated types |
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Default declaration |
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#REQUIRED |
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#IMPLIED (no default value provided) |
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Attribute value (character data) |
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Examples |
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<!ATTLIST MEMO importance (HIGH|MEDIUM|LOW)
"LOW"> |
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<!ATTLIST SIGN signatureFile CDATA #IMPLIED |
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email CDATA #REQUIRED> |
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<!ENTITY Name Entity-Definition > |
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Entity definition |
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Entity value |
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External ID |
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SYSTEM URL |
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PUBLIC Public-identifier URL |
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(External-ID NDATA Name) |
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<!ELEMENT MEMO (TO,FROM,SUBJECT,BODY,SIGN)> |
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<!ATTLIST MEMO importance (HIGH|MEDIUM|LOW) "LOW"> |
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<!ELEMENT TO (#PCDATA)> |
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<!ELEMENT FROM (#PCDATA)> |
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<!ELEMENT SUBJECT (#PCDATA)> |
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<!ELEMENT BODY (P+)> |
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<!ELEMENT P (#PCDATA)> |
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<!ELEMENT SIGN (#PCDATA)> |
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<!ATTLIST SIGN signatureFile CDATA #IMPLIED |
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email CDATA #REQUIRED> |
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<!DOCTYPE MEMO SYSTEM "http://www.
..."> |
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<MEMO importance HIGH> |
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<TO>Jones</TO> |
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<FROM>S.Smith</FROM> |
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<SUBJECT>Project Plan</SUBJECT> |
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<BODY> |
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<P> … </P> |
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<P> … </P> |
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</BODY> |
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<SIGN email SSMITH.CS.STANFORD.EDU> |
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S.
Smith |
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</SIGN> |
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</MEMO> |
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<!ELEMENT novel |
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(preface,chapter+,biography?)> |
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<!ELEMENT preface (paragraph+)> |
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<!ELEMENT chapter
(title,paragraph+,section+)> |
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<!ELEMENT section (title,paragraph+)> |
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<!ELEMENT biography (title,paragraph+)> |
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<!ELEMENT paragraph (#PCDATA|keyword)*> |
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<!ELEMENT title (#PCDATA|keyword)*> |
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<!ELEMENT keyword (#PCDATA)> |
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URI (Universal Resource Identifier) |
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The Web is an information space |
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The URIs are the points in that space |
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URI: name or address that refers to a resource |
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URL (Uniform Resource Locator): URI that
includes explicit instructions on how to access the resource on the
internet |
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XML namespace |
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Set of names used as element types and attribute
names |
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Identified by a URI |
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Universally unique |
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Qualified name |
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A universally unique identifier |
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Syntax: NamespaceName
‘:’ LocalPart |
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A namespace is declared as an attribute
specification of attribute xmlns or an attribute whose prefix is xmlns: |
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Example:
<x xmlns:edi = 'http://ecommerce.org/schema'> |
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<!-- the "edi" prefix is bound to |
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http://ecommerce.org/schema for the "x" |
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element and contents --> |
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</x> |
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A declaration applies to the element where it is
specified and to all elements within the content of that element, unless
overridden by another namespace declaration with the same attribute name |
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Example:
<!-- both namespace prefixes are available |
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throughout --> |
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<bk:book xmlns:bk = 'urn:loc.gov:books' |
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xmlns:isbn = 'urn:ISBN:0-395-36341-6'> |
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<bk:title> Cheaper by the Dozen </bk:title> |
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<isbn:number> 1568491379 </isbn:number> |
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</bk:book> |
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Example of multiple namespace prefixes in an
element |
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<bk:book xmlns:bk = 'urn:loc.gov:books' |
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xmlns:isbn = 'urn:ISBN:0-395-36341-6'> |
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<bk:title> Cheaper by the Dozen </bk:title> |
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<isbn:number> 1568491379 </isbn:number> |
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</bk:book> |
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Example of default namespace in an element |
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<book xmlns = 'urn:loc.gov:books' |
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xmlns:isbn = 'urn:ISBN:0-395-36341-6'> |
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<title> Cheaper by the Dozen </title> |
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<isbn:number> 1568491379 </isbn:number> |
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</book> |
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“XML is like HTML, where you make up your own
tags.” |
|
Provides a uniform method for describing and
exchanging data using the HTTP protocol |
|
HTML enables a universal method for displaying
data |
|
Tag a word to be displayed in bold or italic |
|
XML provides a universal method for describing
data |
|
Declare data to be a retail price, a sales tax,
a book title, ... |
|
Data is made up of characters or unparsed
“entities” |
|
Provides a “syntactic schema” |
|
Provides no means of specifying semantics |
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Notes