Institute of Information Science - University of Miskolc

This is an old revision of the document!

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

<!DOCTYPE uzenet [
        ....
]>

External URI

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

External file

<!DOCTYPE uzenet SYSTEM "filename.dtd">

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:

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

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

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

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

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

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

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

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

<datastorage>
    <pendrive>64MB</pendrive>
</datastorage>

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:

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

<CD>
    <felvetel>Song 1</felvetel>
    <felvetel>Song 2</felvetel>
</CD>

#### Example: An album has at least one title, followed by at least one track title and duration:

```dtd <!DOCTYPE adathordozo [

  <!ELEMENT album (cim+, (dalcim, idotartam)+)>
  <!ELEMENT cim ( #PCDATA ) >
  <!ELEMENT dalcim ( #PCDATA ) >
  <!ELEMENT idotartam ( #PCDATA ) >

]> ```

Syntactically correct XML:

```xml <album>

  <cim>Title 1</cim>
  <cim>Subtitle</cim>
  <dalcim>Track Title 1</dalcim>
  <idotartam>3.42</idotartam>
  <dalcim>Track Title 2</dalcim>
  <idotartam>2.32</idotartam>

</album> ```

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