Institute of Information Science - University of Miskolc

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==== DTD (Document Type Definition) ====

In simple cases, **DTD** is used to describe the syntax of a data structure stored in XML. This DTD can be saved in a file with a ''.dtd'' extension, but it can also be part of the XML document itself.

=== Basic Properties of DTD ==

- Allows for simple **syntactic validation**.

- A descriptive language similar to **extended Backus-Naur form**.

- You can define structures, required sequences, type constraints, and cardinality.
  
If an XML document has a document type declaration, it must be indicated in the file with a special declaration starting with `<!DOCTYPE>`. The type declaration can be **internal** or **external** (referenced via **URI** or a **file**).

=== Internal declaration ===
<sxh>
<!DOCTYPE message [
        ....
]>
</sxh>

=== External URI ===
<sxh>
<!DOCTYPE html PUBLIC "-//W3//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/......dtd">
</sxh>

=== External file ===
<sxh>
<!DOCTYPE message SYSTEM "filename.dtd">
</sxh>

The DTD defines the structure and rules that an XML document must follow to be considered valid.

==== DTD Through Examples ====

Here is a DTD file named ''message_syntax.dtd'' and the corresponding XML on the right:

<sxh>
<!ELEMENT message ( text )>
<!ELEMENT text ( #PCDATA )>
</sxh>

<sxh xml>
<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE message SYSTEM "message_syntax.dtd">

<message>
   <text>Hello XML</text>
</message>
</sxh>

In the XML structure, a **comma** indicates the required order of elements:

<sxh>
<!DOCTYPE studygroup [
        <!ELEMENT group (teacher, student)>
        <!ELEMENT teacher ( #PCDATA ) >
        <!ELEMENT student ( #PCDATA ) >   
]>
</sxh>

<sxh xml>
<studygroup>
    <teacher>Kiss Janos</teacher>
    <student>Gipsz Jakab</student>
</studygroup>
</sxh>

The **pipe symbol `|`** expresses an **either-or** relationship. For example, the storage device can be either a **pendrive** or an **SSD**, but not both:

<sxh>
<!DOCTYPE datastore [
    <!ELEMENT datastorage  (pendrive | SSD)>
    <!ELEMENT pendrive ( #PCDATA ) >
    <!ELEMENT SSD ( #PCDATA ) >   
]>
</sxh>

<sxh xml>
<datastorage>
    <pendrive>64MB</pendrive>
</datastorage>
</sxh>

There are three ways to express **frequency**:

^ Symbol ^ Meaning ^
| ''+''    | The element appears at least once. |
| ''*''    | The element can appear any number of times, including zero. |
| ''?''    | The element appears zero or one time. |

==== Example: A music CD contains **tracks**: ====

<sxh>
<!DOCTYPE datastorage [
    <!ELEMENT CD (recording + )>
    <!ELEMENT recording ( #PCDATA ) >   
]>
</sxh>

<sxh xml>
<CD>
    <recording>Song 1</recording>
    <recording>Song 2</recording>
</CD>
</sxh>
----
==== Example: An album has at least one title, followed by at least one **track title** and **duration**: ====

<sxh>
<!DOCTYPE datastorage [
    <!ELEMENT album (title+, (tracktitle, duration)+)>
    <!ELEMENT title ( #PCDATA ) >
    <!ELEMENT tracktitle ( #PCDATA ) >
    <!ELEMENT duration ( #PCDATA ) >   
]>
</sxh>

**Syntactically correct XML**:

<sxh xml>
<album>
    <title>Title 1</title>
    <title>Subtitle</title>
    <tracktitle>Track Title 1</tracktitle>
    <duration>3.42</duration>
    <tracktitle>Track Title 2</tracktitle>
    <duration>2.32</duration>
</album>
</sxh>

----

==== Example: A library may contain 'books' (zero or more)====

<sxh>
<!DOCTYPE datastorage  [
    <!ELEMENT library (book*) >
    <!ELEMENT book (author, title) >
    <!ELEMENT author ( #PCDATA ) >
    <!ELEMENT title ( #PCDATA ) >   
]>
</sxh>

**Syntactically correct XML**:

<sxh xml>
<library>
    <book >
        <author>Orwell, George</author>
        <title >1984</title >
    </book>
    <book>
        <author>Brown, Dan</author>
        <title >The Da Vinci Code</title >
    </book >
</library>
</sxh>

===== Defining Attributes in DTD =====

If a class has an attribute such as "number of students," it can be specified as follows:

<sxh>
<!ELEMENT class (student *) >
<!ATTLIST class number CDATA #REQUIRED>
</sxh>

Attributes can be:

  * **#IMPLIED**: not mandatory
  * **#REQUIRED**: mandatory
  * **#FIXED**: fixed value

=== Specifying a Default Attribute Value ==

Example:

<sxh>
<!ATTLIST paymentType type CDATA "bankTransfer">
</sxh

The XML could look like this:

<sxh>
<paymentType/> or <paymentType type="bankTransfer">
</sxh>

=== Enumerated Values ===

**Syntax**:
<sxh>
<!ATTLIST element-name attribute-name (eval | eval | ..) default-value>
</sxh>

**DTD Example**:
<sxh>
<!ATTLIST payment type (check | cash) "cash">
</sxh>

**XML Example**:
<sxh xml>
<payment type="check"/> or <payment type="cash"/>
</sxh>

==== Complex DTD Example for a Hypothetical Mail Processing System ====

**Task**: Provide an XML example that satisfies the following DTD:

<sxh>
<!ELEMENT mails (email*, postcard*)>
<!ELEMENT email (address, sender, message?, attachment?)>
<!ELEMENT postcard (address, sender?, message?)>
<!ELEMENT address (name, postalcode, city, country)>
<!ATTLIST address nick CDATA #IMPLIED>
<!ATTLIST sender nick CDATA #IMPLIED>
<!ATTLIST attachment type CDATA #REQUIRED>
<!ATTLIST postcard scanimage CDATA #IMPLIED>
<!ATTLIST sender name CDATA #REQUIRED>
</sxh>

=== A Possible XML for the given DTD ===

<sxh xml>
<mails>
    <email>
        <address nick="Alice">alice@usa.com</address>
        <sender nick="Bob">bob@jp.com</sender>
        <message>Hash code</message>
        <attachment type="text/doc"></attachment>
    </email>
    <postcard scanimage="kep.jpg">
       <address>
         <name>John Doe</name>
         <postalcode>1234</postalcode>
         <city>Miskolc</city>
         <country>Hungary</country>
       </address>
       <sender name="Bob Cat" />
       <message>Happy Name Day</message>
    </postcard>
</mails>
</sxh>

This provides an example of how attributes, including mandatory, optional, and default values, can be defined in a DTD and represented in an XML document.