XML & Database

What is XML?&&&&&&&&

XML stands for EXtensible Markup Language
XML is a markup language much like HTML
XML was designed to describe data
XML tags are not predefined in XML. You must define your own tags
XML uses a Document Type Definition (DTD) or an XML Schema to describe the data
XML with a DTD or XML Schema is designed to be self-descriptive

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The main difference between XML and HTML

XML was designed to carry data.

XML is not a replacement for HTML.
XML and HTML were designed with different goals:

XML was designed to describe data and to focus on what data is.
HTML was designed to display data and to focus on how data looks.

HTML is about displaying information, XML is about describing information.

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XML does not DO anything

XML was not designed to DO anything.

Maybe it is a little hard to understand, but XML does not DO anything. XML is created to structure,

store, and to send information.

The following example is a note to Tove from Jani, stored as XML:

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

The note has a header, and a message body. It also has sender and receiver information. But still, this XML document does not DO anything. It is just pure information wrapped in XML tags. Someone must write a piece of software to send, receive, or display it.

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XML is free and extensible

XML tags are not predefined. You must "invent" your own tags.

The tags used to mark up HTML documents and the structure of HTML documents are predefined. The author of HTML documents can only use tags that are defined in the HTML standard (like , <h1>, etc.).

XML allows the author to define his own tags and his own document structure.

The tags in the example above (like <to> and <from>), are not defined in any XML standard. These tags are "invented" by the author of the XML document.

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XML is a complement to HTML

XML is not a replacement for HTML.

It is important to understand that XML is not a replacement for HTML. In future Web development it is most likely that XML will be used to describe the data, while HTML will be used to format and display the same data.

My best description of XML is: XML is a cross-platform, software and hardware independent tool for transmitting information.

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XML in future Web development

XML is going to be everywhere.

We have been participating in XML development since its creation. It has been amazing to see how quickly the XML standard has been developed, and how quickly a large number of software vendors have adopted the standard.

We strongly believe that XML will be as important to the future of the Web as HTML has been to the foundation of the Web, and that XML will be the most common tool for all data manipulation and data transmission.

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How can XML be Used?&&&&&&&

It is important to understand that XML was designed to store, carry, and exchange data. XML was not designed to display data.

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XML can Separate Data from HTML

With XML, your data is stored outside your HTML.

When HTML is used to display data, the data is stored inside your HTML. With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for data layout and display, and be sure that changes in the underlying data will not require any changes to your HTML.

XML data can also be stored inside HTML pages as "Data Islands". You can still concentrate on using HTML only for formatting and displaying the data.

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XML is used to Exchange Data

With XML, data can be exchanged between incompatible systems.

In the real world, computer systems and databases contain data in incompatible formats. One of the most time-consuming challenges for developers has been to exchange data between such systems over the Internet.

Converting the data to XML can greatly reduce this complexity and create data that can be read by many different types of applications.

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XML and B2B

With XML, financial information can be exchanged over the Internet.

Expect to see a lot about XML and B2B (Business To Business) in the near future.

XML is going to be the main language for exchanging financial information between businesses over the Internet. A lot of interesting B2B applications are under development.

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XML can be used to Share Data

With XML, plain text files can be used to share data.

Since XML data is stored in plain text format, XML provides a software- and hardware-independent way of sharing data.

This makes it much easier to create data that different applications can work with. It also makes it easier to expand or upgrade a system to new operating systems, servers, applications, and new browsers.

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XML can be used to Store Data

With XML, plain text files can be used to store data.

XML can also be used to store data in files or in databases. Applications can be written to store and retrieve information from the store, and generic applications can be used to display the data.

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XML can make your Data more Useful

With XML, your data is available to more users.

Since XML is independent of hardware, software and application, you can make your data available to other than only standard HTML browsers.

Other clients and applications can access your XML files as data sources, like they are accessing databases. Your data can be made available to all kinds of "reading machines" (agents), and it is easier to make your data available for blind people, or people with other disabilities.

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XML can be used to Create new Languages

XML is the mother of WAP and WML.

The Wireless Markup Language (WML), used to markup Internet applications for handheld devices like mobile phones, is written in XML.

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If Developers have Sense

If they DO have sense, all future applications will exchange their data in XML.

The future might give us word processors, spreadsheet applications and databases that can read each other's data in a pure text format, without any conversion utilities in between.

We can only pray that Microsoft and all the other software vendors will agree.

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XML Syntax&&&&&&&

The syntax rules of XML are very simple and very strict. The rules are very easy to learn, and very easy to use.

Because of this, creating software that can read and manipulate XML is very easy to do.

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XML documents use a self-describing and simple syntax.

<?xml version="1.0" encoding="ISO-8859-1"?>

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

The first line in the document - the XML declaration - defines the XML version and the character encoding used in the document. In this case the document conforms to the 1.0 specification of XML and uses the ISO-8859-1 (Latin-1/West European) character set.

The next line describes the root element of the document (like it was saying: "this document is a note"):

<note>

The next 4 lines describe 4 child elements of the root (to, from, heading, and body):

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

And finally the last line defines the end of the root element:

</note>

Can you detect from this example that the XML document contains a Note to Tove from Jani? Don't you agree that XML is pretty self-descriptive?

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All XML elements must have a closing tag

With XML, it is illegal to omit the closing tag.

In HTML some elements do not have to have a closing tag. The following code is legal in HTML:

This is a paragraph

This is another paragraph

In XML all elements must have a closing tag, like this:

This is a paragraph

This is another paragraph

Note: You might have noticed from the previous example that the XML declaration did not have a closing tag. This is not an error. The declaration is not a part of the XML document itself. It is not an XML element, and it should not have a closing tag.

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XML tags are case sensitive

Unlike HTML, XML tags are case sensitive.

With XML, the tag <Letter> is different from the tag <letter>.

Opening and closing tags must therefore be written with the same case:

<Message>This is incorrect</message>

<message>This is correct</message>

All XML elements must be properly nested

Improper nesting of tags makes no sense to XML.

In HTML some elements can be improperly nested within each other like this:

This text is bold and italic

In XML all elements must be properly nested within each other like this:

This text is bold and italic

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All XML documents must have a root tag

The first tag in an XML document is the root tag.

All XML documents must contain a single tag pair to define the root element. All other elements must be nested within the root element.

All elements can have sub elements (children). Sub elements must be correctly nested within their parent element:

<root>

<child>

</child>

</root>

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Attribute values must always be quoted

With XML, it is illegal to omit quotation marks around attribute values.

XML elements can have attributes in name/value pairs just like in HTML. In XML the attribute value must always be quoted. Study the two XML documents below. The first one is incorrect, the second is correct:

<?xml version="1.0" encoding="ISO-8859-1"?>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

<?xml version="1.0" encoding="ISO-8859-1"?>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

The error in the first document is that the date attribute in the note element is not quoted.

This is correct: date="12/11/99". This is incorrect: date=12/11/99.

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With XML, White Space is Preserved

With XML, the white space in your document is not truncated.

This is unlike HTML. With HTML, a sentence like this:

Hello my name is Tove,

will be displayed like this:

Hello my name is Tove,

because HTML strips off the white space.

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With XML, CR / LF is Converted to LF

With XML, a new line is always stored as LF.

Do you know what a typewriter is?. Well, a typewriter is a type of mechanical device they used in the previous century :-)

After you have typed one line of text on a typewriter, you have to manually return the printing carriage to the left margin position and manually feed the paper up one line.

In Windows applications, a new line in the text is normally stored as a pair of CR LF (carriage return, line feed) characters. In Unix applications, a new line is normally stored as a LF character. Macintosh applications use only a CR character to store a new line.

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Comments in XML

The syntax for writing comments in XML is similar to that of HTML.

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There is Nothing Special about XML

There is nothing special about XML. It is just plain text with the addition of some XML tags enclosed in angle brackets.

Software that can handle plain text can also handle XML. In a simple text editor, the XML tags will be visible and will not be handled specially.

In an XML-aware application however, the XML tags can be handled specially. The tags may or may not be visible, or have a functional meaning, depending on the nature of the application.

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XML Elements&&&&&&&&

XML Elements are extensible and they have relationships.

XML Elements have simple naming rules.

XML Elements are Extensible

XML documents can be extended to carry more information.

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Look at the following XML NOTE example:

<note>

<body>Don't forget me this weekend!</body>

</note>

Let's imagine that we created an application that extracted the <to>, <from>, and <body> elements from the XML document to produce this output:

MESSAGE

To: Tove
From: Jani

Don't forget me this weekend!

Imagine that the author of the XML document added some extra information to it:

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

Should the application break or crash?

No. The application should still be able to find the <to>, <from>, and <body> elements in the XML document and produce the same output.

XML documents are Extensible.

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XML Elements have Relationships

Elements are related as parents and children.

To understand XML terminology, you have to know how relationships between XML elements are named, and how element content is described.

Imagine that this is a description of a book:

Book Title: My First XML

Chapter 1: Introduction to XML

What is HTML
What is XML

Chapter 2: XML Syntax

Elements must have a closing tag
Elements must be properly nested

Imagine that this XML document describes the book:

<book>

<title>My First XML</title>

<chapter>Introduction to XML

</chapter>

<chapter>XML Syntax

<para>Elements must have a closing tag</para>

<para>Elements must be properly nested</para>

</chapter>

</book>

Book is the root element. Title, prod, and chapter are child elements of book. Book is the parent element of title, prod, and chapter. Title, prod, and chapter are siblings (or sister elements) because they have the same parent.

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Elements have Content

Elements can have different content types.

An XML element is everything from (including) the element's start tag to (including) the element's end tag.

An element can have element content, mixed content, simple content, or empty content. An element can also have attributes.

In the example above, book has element content, because it contains other elements. Chapter has mixed content because it contains both text and other elements. Para has simple content (or text content) because it contains only text. Prod has empty content, because it carries no information.

In the example above only the prod element has attributes. The attribute named id has the value "33-657". The attribute named media has the value "paper".

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Element Naming

XML elements must follow these naming rules:

Names can contain letters, numbers, and other characters
Names must not start with a number or punctuation character
Names must not start with the letters xml (or XML or Xml ..)
Names cannot contain spaces

Take care when you "invent" element names and follow these simple rules:

Any name can be used, no words are reserved, but the idea is to make names descriptive. Names with an underscore separator are nice.

Examples: <first_name>, <last_name>.

Avoid "-" and "." in names. It could be a mess if your software tried to subtract name from first (first-name) or think that "name" is a property of the object "first" (first.name).

Element names can be as long as you like, but don't exaggerate. Names should be short and simple, like this: <book_title> not like this: <the_title_of_the_book>.

XML documents often have a corresponding database, in which fields exist corresponding to elements in the XML document. A good practice is to use the naming rules of your database for the elements in the XML documents.

Non-English letters like éòá are perfectly legal in XML element names, but watch out for problems if your software vendor doesn't support them.

The ":" should not be used in element names because it is reserved to be used for something called namespaces (more later).

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XML Attributes

XML elements can have attributes in the start tag, just like HTML.

Attributes are used to provide additional information about elements.

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XML elements can have attributes.

From HTML you will remember this: <IMG SRC="computer.gif">. The SRC attribute provides additional information about the IMG element.

In HTML (and in XML) attributes provide additional information about elements:

Attributes often provide information that is not a part of the data. In the example below, the file type is irrelevant to the data, but important to the software that wants to manipulate the element:

<file type="gif">computer.gif</file>

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Quote Styles, "female" or 'female'?

Attribute values must always be enclosed in quotes, but either single or double quotes can be used. For a person's sex, the person tag can be written like this:

or like this:

Double quotes are the most common, but sometimes (if the attribute value itself contains quotes) it is necessary to use single quotes, like in this example:

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Use of Elements vs. Attributes

Data can be stored in child elements or in attributes.

Take a look at these examples:

<lastname>Smith</lastname>

</person>

<sex>female</sex>

<lastname>Smith</lastname>

</person>

In the first example sex is an attribute. In the last, sex is a child element. Both examples provide the same information.

There are no rules about when to use attributes, and when to use child elements. My experience is that attributes are handy in HTML, but in XML you should try to avoid them. Use child elements if the information feels like data.

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My Favorite Way

I like to store data in child elements.

The following three XML documents contain exactly the same information:

A date attribute is used in the first example:

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

A date element is used in the second example:

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

An expanded date element is used in the third: (THIS IS MY FAVORITE):

<note>

<date>

</date>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

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Avoid using attributes?

Should you avoid using attributes?

Here are some of the problems using attributes:

attributes cannot contain multiple values (child elements can)
attributes are not easily expandable (for future changes)
attributes cannot describe structures (child elements can)
attributes are more difficult to manipulate by program code
attribute values are not easy to test against a DTD

If you use attributes as containers for data, you end up with documents that are difficult to read and maintain. Try to use elements to describe data. Use attributes only to provide information that is not relevant to the data.

Don't end up like this ( if you think this looks like XML, you have not understood the point):

<note day="12" month="11" year="99"

to="Tove" from="Jani" heading="Reminder"

body="Don't forget me this weekend!">

</note>

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An Exception to my Attribute rule

Rules always have exceptions.

My rule about attributes has one exception:

Sometimes I assign ID references to elements. These ID references can be used to access XML elements in much the same way as the NAME or ID attributes in HTML. This example demonstrates this:

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

<heading>Re: Reminder</heading>

</note>

</messages>

The ID in these examples is just a counter, or a unique identifier, to identify the different notes in the XML file, and not a part of the note data.

What I am trying to say here is that metadata (data about data) should be stored as attributes, and that data itself should be stored as elements.

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Introduction to DTD&&&&&&&

The purpose of a Document Type Definition is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements.

A DTD can be declared inline in your XML document, or as an external reference.

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Internal DOCTYPE declaration

If the DTD is included in your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:

<!DOCTYPE root-element [element-declarations]>

Example XML document with a DTD: (Open it in IE5, and select view source):

<?xml version="1.0"?>

<!DOCTYPE note [

<!ELEMENT note (to,from,heading,body)>

<!ELEMENT to (#PCDATA)>

<!ELEMENT from (#PCDATA)>

<!ELEMENT heading (#PCDATA)>

<!ELEMENT body (#PCDATA)>

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend</body>

</note>

The DTD above is interpreted like this:

!DOCTYPE note (in line 2) defines that this is a document of the type note.
!ELEMENT note (in line 3) defines the note element as having four elements: "to,from,heading,body".
!ELEMENT to (in line 4) defines the to element to be of the type "#PCDATA".
!ELEMENT from (in line 5) defines the from element to be of the type "#PCDATA"
and so on.....

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External DOCTYPE declaration

If the DTD is external to your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:

<!DOCTYPE root-element SYSTEM "filename">

This is the same XML document as above, but with an external DTD: (Open it in IE5, and select view source)

<?xml version="1.0"?>

<!DOCTYPE note SYSTEM "note.dtd">

<note>

<heading>Reminder</heading>

<body>Don't forget me this weekend!</body>

</note>

And this is a copy of the file "note.dtd" containing the DTD:

<!ELEMENT note (to,from,heading,body)>

<!ELEMENT to (#PCDATA)>

<!ELEMENT from (#PCDATA)>

<!ELEMENT heading (#PCDATA)>

<!ELEMENT body (#PCDATA)>

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Why use a DTD?

With DTD, each of your XML files can carry a description of its own format with it.

With a DTD, independent groups of people can agree to use a common DTD for interchanging data.

Your application can use a standard DTD to verify that the data you receive from the outside world is valid.

You can also use a DTD to verify your own data.

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Introduction to XML Schema

XML Schema is an XML based alternative to DTD.

An XML schema describes the structure of an XML document.

The XML Schema language is also referred to as XML Schema Definition (XSD).

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What You Should Already Know

Before you study the XML Schema Language, you should have a basic understanding of XML and XML Namespaces. It will also help to have some basic understanding of DTD.

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What is an XML Schema?

The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD.

An XML Schema:

· defines elements that can appear in a document

· defines attributes that can appear in a document

· defines which elements are child elements

· defines the order of child elements

· defines the number of child elements

· defines whether an element is empty or can include text

· defines data types for elements and attributes

· defines default and fixed values for elements and attributes

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XML Schemas are the Successors of DTDs

We think that very soon XML Schemas will be used in most Web applications as a replacement for DTDs. Here are some reasons:

· XML Schemas are extensible to future additions

· XML Schemas are richer and more useful than DTDs

· XML Schemas are written in XML

· XML Schemas support data types

· XML Schemas support namespaces

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XML Schema is a W3C Recommendation

XML Schema was originally proposed by Microsoft, but became an official W3C recommendation in May 2001.

The specification is now stable and has been reviewed by the W3C Membership.

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Presentation:

Data and XML

-- Originally designed to isolating content from presentation.

-- Useful in data exchange.

-- Ability to integrate data and documents

-- Designed to communicate content in a flexible and extensible representation.

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XML and database

-- Xml documents can be stored in a relational or objected-oriented database management.

system by translating the documents into relation or objects.

-- Database software can present existing relations or objects in a database as XML.

-- A new DBSM created with a data model based on XML(native XML database).

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Data-Centric (Data- Processing-Oriented or Message-Oriented) Documents:

--Documents that use XML as data transport.

--Documents are characterized by fairly regular structure with many repetitions of those data structures.

--Processing focus on it’s use and exchange byapplications.

-- Desired operation:

searching for combination of elements and data.

modifying, adding or deleting a single element or data.

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--example of data-centric

<CustName>ABC Industries</CustName>

<City>Chicago</City>

</Customer>

Turkey wrench:

Stainless steel, one-piece construction,

lifetime guarantee.

</Description>

</Part>

</Item>

Stuffing separator:

Aluminum, one-year guarantee.

</Description>

</Part>

</Item>

</SalesOrder>

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Document-Centric(Document-Processing-Oriented or Presentation-Oriented) Documents:

--Documents are designed(capture natural language) for human consumption.

--Complex or irregular structure and mixed contents.

--The processing focus on final presentation of information to the end users.

--Desired operation:

retrieving the entire document.

searching for a word.

modifying or reordering a section.

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--Example of document-centric

<Intro>

The <ProductName>Turkey Wrench</ProductName> from <Developer>Full

Fabrication Labs, Inc.</Developer> is <Summary>like a monkey wrench,

but not as big.</Summary>

</Intro>

<Para>The turkey wrench, which comes in both right- and left-

handed versions (skyhook optional), is made of the finest

stainless steel. The Readi-grip rubberized handle quickly adapts

to your hands, even in the greasiest situations. Adjustment is

possible through a variety of custom dials.</Para>

<List>

<Item><Link URL="Order.html">Order your own turkey wrench</Link></Item>

<Item><Link URL="Wrenches.htm">Read more about wrenches</Link></Item>

<Item><Link URL="Catalog.zip">Download the catalog</Link></Item>

</List>

<Para>The turkey wrench costs just $19.99 and, if you

order now, comes with a hand-crafted shrimp hammer as a

bonus gift.</Para>

</Description>

</Product>

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Distinction of Data-Centric and Document-Centric

--Will help you decide which kind of database to use.

--As a general rule, data is stored in traditional database, and documents are stored in

a native XML database or content management system.

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Why use XML database

--XML is useful for data exchange database and applications, especially between different enterprises.

-- XML repositories may meet all needs

-- Easier to represent more complex data than relation in a relational database.

-- XML has some features that similar to objects in object-oriented programming language.

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