CSE 3241: XML Extensible Markup Language (Ch. 12)

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CSE 3241: XML

Extensible Markup Language

(Ch. 12)

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Topics

 Structured, Semistructured, and Unstructured Data

 XML Hierarchical (Tree) Data Model

 XML Documents

 DTD (Document Type Definition)

 XML Schema

 Storing and Extracting XML Documents from Databases

 XML Languages

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Structured, Semistructured, and Unstructured Data

 Structured data

◦ Represented in a strict format

◦ Example: information stored in databases

 Semistructured data

◦ Has a certain structure

◦ Not all information collected will

have identical structure

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Structured, Semistructured,

and Unstructured Data (cont’d.)

◦ Self-describing data

 Schema information mixed in with data values

 May be displayed as a directed graph

 Labels or tags on directed edges represent:

 Schema names

 Names of attributes

 Object types (or entity types or classes)

 Relationships

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Unstructured Data

 Limited indication of the of data document that contains

information embedded within it

 HTML documents

◦ Do not include schema information about type of data

 Static HTML page

◦ All information to be displayed

explicitly spelled out as fixed text in

HTML file

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Unstructured Data

 HTML uses a large number of predefined tags

◦ Tag

 Text that appears between angled brackets:

<...>

◦ End tag

 Tag with a slash: </...>

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Projects Proj X

Proj Y Worker

s

Worker s

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Semistructured Data

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SemiStructured Data: XML

 Data sources

◦ Database storing data for Internet applications

 Hypertext documents

◦ Common method of specifying

contents and formatting of Web

pages

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What is XML?

 XML – The eXtensible Markup Language

 What’s a Markup Language?

◦ Language used to annotate a document for some purpose

◦ Uses tags that are distinguished from the content of the document to provide that annotation

◦ HTML (HyperText Markup Language) and LaTeX

 Both examples of document publishing languges

 Tags used to indicate formatting

 Tags follow a defined structure to keep them

separate from the content of the document ¹⁰

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What is XML?

 XML provides a framework to define a structure for data

◦ An XML document is a collection of related data items

◦ Document is “marked up” with tags known as elements

 Elements are used to provide structure to the data

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XML Hierarchical (Tree) Data Model

 Elements and attributes

◦ Main structuring concepts used to construct an XML document

 Complex elements

◦ Constructed from other elements hierarchically

 Simple elements

◦ Contain data values

 XML tag names

◦ Describe the meaning of the data

elements in the document

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XML Hierarchical (Tree) Data Model (cont’d.)

 XML attributes

◦ Describe properties and

characteristics of the elements (tags) within which they appear

 May reference another element in another part of the XML

document

◦ Common to use attribute values in

one element as the references

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The XML Data Model

 Attributes vs.

Elements

◦ Data can be stored as the contents of an

element OR as an

attribute of an element

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<?xml version=“1.0”

standalone=“yes”?>

<Name>Product X</Name>

<Location>Bellaire</Location>

<Dept_no>5</Dept_no>

…

</Projects>

Why pick one over the other?

Best practice:

Attributes at describe/modify the element Elements to hold the actual data values

Much like in HTML:

Element (tag) contents are the data to be displayed Attributes (generally) modify/describe how it is to be displayed

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What does XML have to do with databases?

 Recall: What is a database?

◦ A logically coherent collection of data with some specific meaning that has been

designed for a specific purpose.

 Structured and semi-structured data files vs.

database?

◦ More practically, XML is used as a data exchange framework

 Moving data from one application to another, from one database to another

 Taking data from a database and turning it into a website, a report, or other human readable

document

◦ Even some implementations of “XML native” DBs

 XML as the “back end” storage instead of relations

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The XML Data Model

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 XML uses a hierarchical model

◦ Also known as a tree model Documents can be

represented as trees

Each simple element contains one data value

Leaves of the tree

Complex elements can contain multiple child elements

Internal nodes of the tree

Each complex element can belong to one complex parent element

Parent node of the tree

One root element contains everything else

Root of the tree

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A sample XML tree

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•

Internal nodes are complex elements

•

Leaf nodes are simple elements

•

The root node is the root element

•

Root element

contains all other elements within it

Projects

Project Id=“1”

Name Location Dept_no Workers

Ssn Last_name Hours Ssn Hours

Worker Worker

“Product X” “Bellaire” “5”

“123456789” “Smith” “32.5” “453453453” “15.5”

Project Project

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A sample XML tree

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<?xml version=“1.0” standalone=“yes”?

>

<Last_name>Smith</LastName>

</Worker>

</Worker>

</Workers>

</Project>

…

</Projects>

Projects

Project Id=“1”

Name Location Dept_no Workers

Ssn Last_name Hours Ssn Hours

Worker Worker

“Product X” “Bellaire” “5”

“123456789” “Smith” “32.5” “453453453” “15.5”

Project Project

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A sample of XML

<?xml version=“1.0” standalone=“yes”?>

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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XML Declaration

<Location>Bellaire</ Location >

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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root element

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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Beginning of root element

End of root element root element

<?xml version=“1.0”

standalone=“yes”?>

<Location>Bellaire</

Location >

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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First child element of root

(Other child elements possible in here – do not even need to be “Project”

elements necessarily)

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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The first Project element has an attribute named number

with a value of “1”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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First child element of Project element where id=“1”

Simple element with a name of “Name” and a value of “Product X”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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Second child element of Project element where id=“1”

Simple element with a name of “Location” and a value of “Bellaire”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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Third child element of Project element where id=“1”

Simple element with a name of “Dept_no” and a value of “5”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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Fourth child element of Project element where id=“1”

Complex element with a name of “Workers”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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A sample of XML

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First child element of Projects/

Project[number=“1”]/

Workers

Complex element with a name of “Worker”

</Worker>

</Worker>

</Workers>

</Project>

….

</Projects>

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XML Hierarchical (Tree) Data Model (cont’d.)

 Tree model or hierarchical model

 Main types of XML documents

◦ Data-centric XML documents

◦ Document-centric XML documents

◦ Hybrid XML documents

 Schemaless XML documents

◦ Do not follow a predefined schema of element names and corresponding

tree structure

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XML Document Types – Data Centric XML

 Data-centric XML

◦ Highly structured

◦ Many small data items

◦ Often used for data exchange purposes

 Transfer data from one system to another

◦ Also used to create web pages dynamically from databases

◦ Generally follow a schema document that determines their structure

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XML Document Types – Document-Centric XML

 Few structural elements

 Large amounts of text

◦ Articles, blog entries, books

 May have a schema document, but not required

◦ Schema may be very limited in semantics

 What’s a title?

 What’s a chapter?

 What’s a paragraph?

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 Hybrid XML

◦ Some parts are highly structured

◦ Some parts mostly blocks of text and/or unstructured

◦ May or may not have a predefined schema

 Schemaless XML documents

◦ Semi-structured documents without a predefined schema

◦ Denoted by the attribute

‘standalone=“yes”’ in the XML declaration on the top line

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Valid XML

 An XML document is considered valid if:

◦ It is well-formed

◦ And…

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To be continued after this definition…

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Well-formed XML

 An XML document is well-formed when it follows certain conditions:

◦ It must start with an XML declaration line:

<?xml version=“1.0” standalone=“yes”?>

◦ It must form a tree:

 Must start with a single root element

 Every child element must have start and end tags that are contained completely within a parent

element:

Good Bad

</child> </parent>

</parent> </child>

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Valid XML

 An XML document is considered valid if:

◦ It is well-formed, and …

◦ It follows a particular schema in a standard definition language

 A DTD document (Document Type Definition)

 An XML schema document

◦ DTDs are the original, older technology

◦ XML schema documents are the “new”

hotness

 First published in 2001

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DTD – Document Type Definition

 Original method of specifying a schema definition

◦ Still in widespread use

 A very simple schema definition language

◦ Each possible element in the document is defined

 What children must it have?

 What children can it (optionally) have?

 What kinds of attributes can/must it have?

 If it is a leaf element, what kinds of values

can it have?

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XML Documents, DTD, and XML Schema (cont’d.)

 Notation for specifying elements

 XML DTD

◦ Data types in DTD are not very general

◦ Special syntax

 Requires specialized processors

◦ All DTD elements always forced to follow the specified ordering of the document

 Unordered elements not permitted

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A sample XML document and DTD

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<?xml version=“1.0” standalone=“no”?>

<!DOCTYPE Projects SYSTEM “proj.dtd”>

<Location>Bellaire</Location >

</Worker>

</Worker>

</Workers>

</Project>

…

</Projects>

We declare that we want to use a DTD by Putting the DOCTYPE declaration at the top of our XML file

!DOCTYPE keyword Projects

The name of our DTD’s root node SYSTEM

indicating that this is an external DTD

“proj.dtd”

the filename (or URL)

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A sample XML document and DTD

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</Worker>

</Worker>

</Workers>

</Project>

…

</Projects>

<!ELEMENT Projects (Project+)>

<!ELEMENT Project (Name, Location, Dept_no?, Workers)>

<!ATTLIST Project number ID

#REQUIRED>

<!ELEMENT Name (#PCDATA)>

<!ELEMENT Location (#PCDATA)>

<!ELEMENT Dept_no (#PCDATA)>

<!ELEMENT Workers (Worker*)>

<!ELEMENT Worker (Ssn, Last_name?, First_name?, Hours)>

<!ELEMENT Ssn (#PCDATA)>

<!ELEMENT Last_name (#PCDATA)>

<!ELEMENT First_name (#PCDATA)>

<!ELEMENT Hours (#PCDATA)>

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#REQUIRED>

A sample DTD

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root element comes first

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#REQUIRED>

A sample DTD

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Name of element

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#REQUIRED>

A sample DTD

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List of children

Regular expression-like syntax:

+ – indicates 1 or more of this child

* – indicates 0 or more of this child

? – indicates 0 or 1 of this child No symbol – indicates exactly one child

So this indicates 1 or more Project children

are required

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#REQUIRED>

A sample DTD

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List of children

This indicates that Dept_no is an optional

field, but there can be only one of them

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#REQUIRED>

List of children

A sample DTD

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#REQUIRED>

List of children

A sample DTD

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#REQUIRED>

A sample DTD

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Project has an attribute named

“number”

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#REQUIRED>

A sample DTD

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“number”

It’s “type” is a unique ID

This can be used to refer to this child by other elements – like a primary key

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#REQUIRED>

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“number”

It’s “type” is a unique ID

This can be used to refer to this child by other elements – like a primary key

And this attribute ID must exist on all

Project children

A sample DTD

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#REQUIRED>

A sample DTD

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Name is a “leaf node”

#PCDATA means that it holds

“parsed character data”

It will contain a value of some kind between its start and end tag (even an empty value counts as a value for the DTD)

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A sample XML document and DTD

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</Worker>

</Worker>

</Workers>

</Project>

…

</Projects>

#REQUIRED>

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DTD Limitations

 Data types in DTD are not general

◦ Child nodes hold PCDATA values – strings

◦ DTD has its own syntax

 Need to write a special parser for it

 Can’t leverage existing XML parsers to do DTD parsing

◦ All elements must follow the ordering laid out

 Unordered elements not allowed

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