XML DTD

DTD

DTD is called Document Type Definition .
DTD defines the structure of the XML file.
DTD is used to define user tags that is going to be used in the XML file.
DTD is also used to define attributes for user defined tags.
Basically, DTD is to validate the XML file.
Validation of XML file meaning is that to check that XML file is written according to DTD.
User defined tag is also called element of the XML file.
Synatx for defining DTD is
```
  <!DOCTYPE . . .  >
```
DTD starts with left angle bracket (<) and closes with right angle bracket (>).
After left angle bracket, an exclamation mark (!) is used.
After exclamation mark (!), the word DOCTYPE is written.

Defining the root element (or tag) in DTD

Root tag of XML file is defined in DTD by writting root tag name after DOCTYPE.
For example, to write a root tag facultydetail in DTD. Following is the DTD:
```
  <!DOCTYPE FacultyDetail >
```
Further, to define other elements within the root element. Use following DTD:
A Faculty element is mentioned in the root element FacultyDetail .
Now suppose, you want three information (FacultyName, Subject and City) related to faculty in the Faculty element.

Following is DTD code:

  <!DOCTYPE FacultyDetail [ 
 <!ELEMENT Faculty (FacultyName, Subject, City)>
  ]>

Now, furhter each three elements is defined with its type of content.

See the following DTD code:

  <!DOCTYPE FacultyDetail [ 
 <!ELEMENT Faculty (FacultyName, Subject, City)>
 <!ELEMENT FacultyName (#PCDATA)>
 <!ELEMENT Subject (#PCDATA)>
 <!ELEMENT City (#PCDATA)>
  ]>

Here, #PCDATA is used to define type of the content of the element.
#PCDATA defines that content of element is Parsed Character Data .
Remember, DTD is closed using symbol ]> .
Closing bracket ] is used to indicate end of the root element FacultyDetail .
Right angle bracket > after closing bracket ] is used to indicate end of DOCTYPE .

Explanation of DTD example

In the above DTD example, one root element FacultyDetail is defined.
Within root element FacultyDetail , one element Faculty is defined.
Within Faculty element, three elements FacultyName, Subject and City are defined.
Then each three elements is futher defined with its type of content which is Parsed Character Data .
Parsed Character Data is defined using the keyword #PCDATA .

Full XML program with corresponding DTD

Example: Write XML file with corresponding DTD for faculty information.

<?xml version="1.0" encoding = "UTF-8"?>
<!DOCTYPE FacultyDetail [ 
<!ELEMENT Faculty (FacultyName, Subject, City)>
<!ELEMENT FacultyName (#PCDATA)>
<!ELEMENT Subject (#PCDATA)>
<!ELEMENT City (#PCDATA)>
]>

<FacultyDetail>
	<Faculty>
		<FacultyName> Abhay Sahu </FacultyName>
		<Subject> General Knowledge </Subject>
		<City> Kanpur </City>
	</Faculty>
</FacultyDetail>

The above XML program has an internal DTD that defines one root element and other elements of the XML file.

External DTD

In case of external DTD, DTD is written in a separate file with file extension .dtd.
Then .dtd file is included in the XML file.

See the following example.
Suppose name of separate DTD file is fac.dtd as given below:

<!ELEMENT Faculty (FacultyName, Subject, City)>
<!ELEMENT FacultyName (#PCDATA)>
<!ELEMENT Subject (#PCDATA)>
<!ELEMENT City (#PCDATA)>

Now this file is included in the following XML file:

<?xml version="1.0" encoding = "UTF-8"?>
<!DOCTYPE FacultyDetail SYSTEM "fac.dtd">
<FacultyDetail>
	<Faculty>
		<FacultyName> Abhay Sahu </FacultyName>
		<Subject> General Knowledge </Subject>
		<City> Kanpur </City>
	</Faculty>
</FacultyDetail>

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