Introduction to Document Type Definition (DTD)(Basic)

Introduction to DTD

 

● Seperate file from XML document 

● Can be embedded within the XML File 

● It allows to validate the contents of Xml document 

● It contains rules that apply to xml data 

● Extention : .dtd

 

Why do we need DTD

● Replace the use of resusable pieces of text between 2  XML documents
● To verify the XML document
● To meet the constraints

Document Type Declaration

● A DOCTYPE declaration is an XML document specifies a referance to DTD file.
● <!DOCTYPE rootelement “a.dtd”>
● 2 types:
   – Internal DTD
   – External DTD
        ● System DTD
        ● Public DTD

Internal DTD

● The contents of the DTD are inside an XML document.
● Eg:-
    – <?xml version=”1.0”?>
    <!DOCTYPE myBook
        [ <!ELEMENT book_name (#PCDATA) > ]>

<myBook>
<book_name> XML </book_name>
</myBook>

● No need to provide a seperate name for the DTD(contents are inside XML file)

External DTD

● Provide a reference to the DTD  inside XML document.
● Allow as to define DTD once. Refer to any number of XML documents
● It reduce the size of the XML documents.


<?xml version=”1.0”?>
 <!DOCTYPE myBook SYSTEM “mybook.dtd”>

<myBook>      
<book_name> XML </book_name>
</myBook>

● <!ELEMENT book_name (#PCDATA)>

● System DTD: it has existance and relevance only in given text. Available in same computer or same network.

● Public DTD: has usage beyond a single XML document. It is located on a different computer
 accessible via internet as a URL.

Element Type Declaration

● If we want to associate a DTD with an XML document, we need to declare all the elements that we would add in XML document. <!ELEMENT book_name (#PCDATA)> ● book_name: element name is generic identifier ● #PCDATA : data type content specification

Element Content Models

● Sequence, Occurrences, Choice
● Empty, Any, Mixed Content

Sequence

● <!ELEMENT address(street, region, postal-code)
  <!ELEMENT street (#PCDATA)>
  <!ELEMENT region (#PCDATA)>
  <!ELEMENT postal-code (#PCDATA)>

 

● Specified using pipe(|) character
<!ELEMENT guest(name, beverage)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT beverage tea|cofee>

Occurences

● +  :  The element can occure one or more times
● *  :  The element can occure zero or more times
● ?  :  The element can occure zero or one times

+Example












● <!ELEMENT book(Chapter+)>
● <!ELEMENT book (Chapter, author)+)>

* Exanple

 <!ELEMENT organisation (emp*)>
<organisation>
 <emp> ramesh </emp>
<emp> suresh </emp>
</organisation>

<organisation></organisation>

? Example

 <!ELEMENT organisation(emp?)>
<organisation>
 <emp>ramesh</emp>
  </organisation>
<organisation></organisation>

Empty, any and mixed content

● Element must be empty, can contain mixed content or can contain any content.
● Empty elements
● <!ELEMENT name EMPTY>
   <name></name> </name>

Mixed Content

● An element can contain either some text or other sub-elements.
● <!ELEMENT mybooks(book | #PCDATA) *>
 <mybooks>book1 name
 <book>Computer network</book>
</mybooks>

Any

● An element declared with ANY type can contain any content, including PCDATA, sub-elements, combinations of the 2 or empty elements

● <!ELEMENT mybook ANY>
● <mybook>XML related tech
 <book></book>
<book>XML 1 </book>

Attribute Declaration

● ATTLIST keyword is used.
● <!DOCTYPE email [
 <!ELEMENT email(message)>
 <!ELEMENT message (#PCDATA)>
<!ATTLIST message from CDATA #condition>]>

<email>
<message from=”Mr.x” to=”Mr. Y” > hello </message>
</email>

Limitations of DTD

● Non XML syntax
 ● One DTD per XML
● Weak data typing
● No inheritance
● Overridding a DTD : internal can override external DTD
● No DOM support

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