Introduction to XML and Databases

Introduction to XML
Kristian Torp
Department of Computer Science
Aalborg University
people.cs.aau.dk/˜torp
torp@cs.aau.dk
November 3, 2015
daisy.aau.dk
Kristian Torp (Aalborg University) Introduction to XML November 3, 2015 1 / 42

Outline
1 Introduction
2 Anatomy of an XML Document
Document Prolog
Elements
Attributes
Entities
Complete XML Document
3 Well-Formed and Valid XML Documents
4 Summary

Learning Goals
Goals
Know the basic differences between a table and an XML document
Know the different representations of an XML document
Know the basic parts of an XML document
Know the goals of designing XML
Know data centric from document centric
Be able to construct your own basic XML documents

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Text Files, (a Dejá Vu?)
Example (A Text File)
P4 OOP 3 Object−oriented programming
P2 DB 7 Databases including SQL
Open Questions
What does the columns mean?
When does white space matter?
What are the types of the columns?
Note
No metadata what so ever
Need additional information to parse the text file!
Could be a human looking at the file
Lowest common denominator a CSV file

A First Look
Example (Table Look)
Id Name Semester Desc
P4 OOP 3 Object-oriented programming

A First Look
Example (Table Look)
Id Name Semester Desc
P4 OOP 3 Object-oriented programming
Example (XML Look)
<?xml version=” 1.0 ” ?>
<!DOCTYPE coursecatalog SYSTEM ” coursecatalog . dtd ”>
<coursecatalog>
<course cid= ’P4 ’>
<name>OOP</name>
<semester>3</ semester>
<desc>Object−oriented programming</ desc>
</ course>
<name>DB</name>
<desc>Databases including SQL</ desc>
</ course>
</ coursecatalog>

A Second Look
Example (XML Look (again))
<coursecatalog>
<name>OOP</name>
</ course>
<name>DB</name>
</ course>
</ coursecatalog>

A Second Look
Example (XML Look (again))
<coursecatalog>
<name>OOP</name>
</ course>
<name>DB</name>
</ course>
</ coursecatalog>
Example (Tree Look)
/coursecatalog
course
id=4 name:OOP sem:3 dsc
course
id=2 name:DB sem:7 dsc

Something Well Known?
Example (XHTML)
<?xml version=” 1.0 ” encoding=”UTF−8” ?>
<!DOCTYPE html PUBLIC ” −//W3C/ / DTD XHTML 1.0 T r a n s i t i o n a l / / EN”
” h t t p : / /www.w3. org /TR/ xhtml1 /DTD/ xhtml1−t r a n s i t i o n a l . dtd ”>
<html xmlns=” h t t p : / /www.w3. org /1999/ xhtml ”>
<head>
< t i t l e >A Simple XHTML Document</ t i t l e >
</ head>
<body>
Hello XHTML!
</ body>
</ html>
[Source: examples/xhtml_simple.xhtml]
XHTML versus HTML
XHTML is a cleaned-up version of HTML
Looks a lot like HTML
Much stricter requirements to XHTML than to HTML

Data versus Document Centric
Example (Data Centric)
<rows>
<row>
<name>Hans</name>
<address>Denmark</ address>
</ row>
<row>
<name>Marge</name>
<address>Sweden</ address>
</ row>
</ rows>
Example (Document Centric)
< l y r i c >
Is i t getting better?
Or do you f e e l the same?
W i l l i t make i t easier on you now?
You got someone to blame
You say
One love
One l i f e
</ l y r i c >
Data Centric
Database table like
Content in leafs
Inﬂexible, but simple
Document Centric
Free format (almost)
Mixed content
Flexible, but complex

Goals of XML
Goals
XML shall be straight forwardly usable over the Internet
XML shall support a wide variety of applications
XML shall be compatible with SGML
SGML = Standard Generalized Markup Language
Easy to write programs which process XML documents
Keep the number of optional features low (0)
XML documents should be reasonably clear
The XML design should be prepared quickly
The design of XML shall be formal and concise
XML documents shall be easy to create
[Source: www.w3.org/TR/REC-xml/]

XML Family of Products
Products
Core
The basic XML recommendation
Add-ons
DTD, XML Namespace, XPath, XLink, XPointer, XQuery, etc.
Focus on layout
CSS, XSLT, and XSL-FO
XML Applications
XHTML, DocBook, SVG, XForms, etc.
XML Applications
Web Content Syndication: RSS (www.rssboard.org)
Education: SCORM for teaching material (www.scorm.com)
Document metadata: Dublin Core (www.dublincore.org)

Summary: Introduction
Main Points
An XML document compared to a text ﬁle
More readable (without help)
More complicated to handle (if you are familiar with content)
Higher space usage
Data and metadata embedded in the same document
Markup and content clearly separated
An XML document can be represented in two ways
Textual structure
Tree structure
The goals of the XML design were made in an Internet age!
There is a very large set of XML technologies and applications
Note
XML and databases are not competing technologies
XML is not a replacement of HTML

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Main Parts of an XML Document
Concepts
Document prolog
Elements
A root
Attributes
Entities
Example (XML Document)
<!DOCTYPE coursecatalog
SYSTEM ” coursecatalog . dtd ” [
<!ENTITY prg ” programming ”>
<! ENTITY sql ”SQL”> ]>
<coursecatalog>
<course id=”P4”>
<name>OOP</name>
<desc>Object−oriented &prg ;</ desc>
</ course>
<name>DB</name>
<desc>Databases including &sql ;</ desc>
</ course>
</ coursecatalog>
Note
Elements more ﬂexible than attributes
XML supports UTF out-of-the box

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Document Prolog
Example
<coursecatalog>
Consists of
Version number and text encoding
Document type deﬁnition declaration
Instruction to the XML processor
Root element of the XML document

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Elements
Example
Start tag <state> or <course>
State tag with attributes <state id=”1” abbr=”GA”>
End tag </state>
Element with content <state>Georgia</state>
Empty element <state/>
Empty element with attributes <state id=”1” abbr=”GA”/>
Case matters <state> <State> <STaTE>
Consists of
Start tag
Some content called character data
End tag

Elements, cont.
Rules
Start tag must be before end tag
An elements start and end tag must have the same parent
Wrong: <state><city></state></city>
Right: <state><city></city></state>
Content
Simple <outer><one>stuff</one></outer>
Mixed content <outer>More <one>stuff</one></outer>
Tag versus Element
<msg>Hello World</msg>
Element: <msg>Hello World</msg>
Tag: msg

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Attributes
Example
<state id=”1” abbr=”GA”>
<country id=”DK”date=”2006−02−01”>
Consists of
Name/value pairs
Note
Attributes cannot stand alone
Only start tags can have attributes
There can be any number of attributes
Attribute names must be unique <state id=”GA”id=”GE”>
Attribute values must be in quotes <state id=GA>

Elements versus Attributes
Example (Elements versus Attributes)
<box height=” 20 ”
width=” 20 ”
depth=” 30 ”
u n i t =”cm”>
<content>S t u f f</ content>
</ box>
<box>
<height>
<scalar>20</ scalar>
cm
</ height>
<width>
</ width>
<depth>
</ depth>
</ box>
Note
Attributes can always be converted to elements
Elements can sometimes be converted to attributes

Elements versus Attributes, cont.
Example (Elements versus Attributes)
<box>
<height>
</ height>
<width>
</ width>
<depth>
</ depth>
</ box>
<box>
<height u n i t =”cm”>20</ height>
<width u n i t =”cm”>20</ width>
<depth u n i t =”cm”>30</ depth>
</ box>
Note
Attributes good for identify, units and so on
Elements good if variable number of “stuff”

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Entities
Example
<! ENTITY company ”XML Lovers Inc.”>
<! ENTITY sql ”SQL”>
Purpose
To make XML document easier to maintain
Recurring text
Are place holders for content (abbreviations)
Types
Parameter entities used in DTD
General entities used in the XML document itself
There are a lot of details about entities!

Using Entities
Example
<!DOCTYPE coursecatalog
SYSTEM ” coursecatalog . dtd ” [
<!ENTITY prg ” programming ”>
<! ENTITY sql ”SQL”> ]>
<coursecatalog>
<name>OOP</name>
<desc>Object−oriented &prg ;</ desc>
</ course>
<name>DB</name>
<desc>Databases including &sql ;</ desc>
</ course>
</ coursecatalog>
[Source: examples/coursecatalog_with_entity.xml]
Note
The entities prg and sql

More Entity Examples
Entity Types
Predefined character entities amp = & gt = >
Usage: <msg>Hello & and ></msg>
Numbered character entities #145 = æ
Usage: <msg>This is a Danish letter </msg>
External entities definition is in another file
Internal entities
Unparsed entity <!ENTITY logo SYSTEM ”logo.gif”NDATA gif>
Note
There are a lot of details about entities!

Various Comments on XML Documents
Comments
Are always in UTF
Whitespace is preserved (not the case in HTML)
Carriage return and line feed converted to line feed
Weird when used to MS Windows
This is a comment <!−−a comment in XML −−>
Example (Comments in XML)
<?xml version=” 1.0 ”>
<doc>
<!−− A comment −−>
<row> </ row>
<row> <!−− Another comment −−> </ row>
</ doc>

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

First Design
Example (1-n Relationship)
<order −db>
<orders>
<order id=” 117 ”>
<customer−name>Ann</ customer−name>
</ order>
<order id=” 341 ”>
<customer−name>Jim</ customer−name>
</ order>
</ orders>
<orderlines>
<o r d e r l i n e id=” 117 ” line −no=” 1 ”>
<description>pizza</ description>
<quantity>1</ quantity>
<price −each>10.50</ price −each>
</ o r d e r l i n e>
</ orderlines>
</ order −db>
Note
Too much ﬁrst normal form, does not use tree hierarchy

Second Design
Example (1-n Relationship)
<order −db>
<orders>
<order id=”O117”>
<customer−name>Ann</ customer−name>
<orderlines>
<o r d e r l i n e line −no=” 1 ”>
<description>pizza</ description>
<quantity>1</ quantity>
<price −each>10.50</ price −each>
</ o r d e r l i n e>
</ orderlines>
</ order>
<order id=”O341”>
<customer−name>Jim</ customer−name>
</ order>
</ orders>
</ order −db>
Note
All information related to single order is stored together

Summary: Anatomy
Main Points
Elements
One is the root
Attribute
Limited set
Entities
Similar to a macro
There are many details
The prolog
Note
In doubt element or attribute? Pick element
Remember good comments, for humans!

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Non Well-Formed XML Document
Example (Missing Root)
<name>OOP</name>
<desc>Object−oriented Prog .</ desc>
</ course>
<name>DB</name>
</ course>

<name>OOP</name>
</ course>
<name>DB</name>
</ course>
Example (Nesting Wrong)
<person ssn=” 43 ”>
<name>< f i r s t >James</ f i r s t > <l a s t>Bond</name></ l a s t>
<job>agent</ job>
</ person>

<name>OOP</name>
</ course>
<name>DB</name>
</ course>
Example (Nesting Wrong)
<person ssn=” 43 ”>
<name>< f i r s t >James</ f i r s t > <l a s t>Bond</name></ l a s t>
<job>agent</ job>
</ person>
Example (Missing Quotes)
<person ssn=43>
<name> . . . </name>
</ person>

Well-Formed XML and Valid Document
Well-Formed XML Document
All XML elements must have a closing tag
Empty elements are allow
Tags must be properly nested
Start and end tag must have the same parent
The XML document must have a root tag
Attribute values must be quoted
Valid XML Document
Is well-formed
Adheres to the rules of the speciﬁed DTD or XML Schema
Similar to a schema for a table, e.g., types and integrity constraints

Well-Formed and Valid
XML Documents
Well-Formed XML Documents
Valid XML Documents

Summary: Well-Formed and Valid
Main Points
Well-formed XML document
Structure must adhere to certain rules
Valid XML document
Types and constraints must match a schema (DTD or XML Schema)
Not covered in this lecture, more to come later
Note
Tools check if documents are well-form and valid
The well-formedness is a huge plus over “ﬂat” ﬁles

Outline
1 Introduction
Document Prolog
Elements
Attributes
Entities
4 Summary

Why XML?
Many Good Reasons
Open
Speciﬁcations available to all
Platform neutral
Runs on Apple, Linux, Unix, Windows, . . .
Vendor neutral
Competition among vendors
Standard
Changes done in open forums
Note
XML has support for checking structure/types/integrity constraints
DTD and XML Schema
XML has support for querying text documents
XPath and XQuery

Data vs. Document Centric
Data Centric
Database designer
Does not use document order
Only content at leaf level
Simple
Rigid
Example: Extract RDBMS
Document Centric
Text author
Document order, e.g., for
chapters ﬁgure no
Mixed content
Complex
Flexible
Examples: DocBook, XHTML

XML vs. DBMS
RDBMS
Structured data
Unordered
Flat information
Native format
Very compact format
SQL
Fine-grained modiﬁcations
Bad data exchange
Integrity via SQL DDL
Supports data types
Extreme data volumes
XML
Structured and unstructured
Ordered
Hierarchical information
Standard format
Very verbose format
XPath and XQuery
Coarse-grained modiﬁcations
Excellent data exchange
Integrity via XML Schema (DTD)
Supports data types
Large data volumes

Additional Information
Web Sites
W3C Schools free online tutorials www.w3schools.com.
Quite good for getting an overview of the various XML technologies.
Interactive XML Tutorials www.xmlzoo.net.
Covers several parts of XML
The Annotated XML 1.0 Speciﬁcation
www.xml.com/axml/testaxml.htm.
The XML 1.0 speciﬁcation with a lot of comments.
W3C XML recommendations www.w3.org.
The place to go if you want all the details.
Altova’s home page (maker of XMLSpy) www.altova.com. If you are
looking for a good XML tool.
IBM developerWorks overview “New to XML”
www.ibm.com/developerworks/xml/newto/
Many links to additional information.

Introduction to XML and Databases

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