A Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.
A DTD can be declared:
- inside XML
- external reference
DTD inside:
DTD external
Why is DTD useful
- With a DTD, each of your XML files can carry a description of its own format.
- With a DTD, independent groups of people can agree to use a standard 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.
Bulding blocks in XML
- Elements
- Attributes
- Entities
- PCDATA
- CDATA
The following entities are predefined in XML:
| Entity References | Character |
| < | < |
| < | < |
| & | & |
| " | " |
| ' | ' |
PCDATA
PCDATA means parsed character data.
PCDATA is text that will be parsed by a parser. The text will be examined by the parser for entities and markup.
CDATA
CDATA means character data.
CDATA is text that will not be parsed by a parser. Tags inside the text will NOT be treated as markup and entities will not be expanded.
Elements
Declaring<!ELEMENT element-name category>
or
<!ELEMENT element-name (element-content)>
Empty elements
<!ELEMENT br EMPTY>
Elements with PCDATA
<!ELEMENT from (#PCDATA)>
Element with children
<!ELEMENT note (to,from,heading,body)>
When children are declared in a sequence separated by commas, the children must appear in the same sequence in the document. In a full declaration, the children must also be declared, and the children can also have children. The full declaration of the "note" element is:
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
One occurence
<!ELEMENT note (message)>
One and more
<!ELEMENT note (message+)>
Zero and more
<!ELEMENT note (message*)>
Zero or one
<!ELEMENT note (message?)>
Either/ or content
<!ELEMENT note (to,from,header,(message|body))>
Mixed content
<!ELEMENT note (#PCDATA|to|from|header|message)*>
Attributes
Declaring attributes<! ATTLIST element-name attribute-name attribute-type default-value>
Example:
<!ATTLIST payment type CDATA "check">
The attribute-type can be one of the following:
| Type | Description |
| CDATA | The value is character data |
| (en1|en2|..) | The value must be one from an enumerated list |
| ID | The value is a unique id |
| IDREF | The value is the id of another element |
| IDREFS | The value is a list of other ids |
| NMTOKEN | The value is a valid XML name |
| NMTOKENS | The value is a list of valid XML names |
| ENTITY | The value is an entity |
| ENTITIES | The value is a list of entities |
| NOTATION | The value is a name of a notation |
| xml: | The value is a predefined xml value |
The default-value can be one of the following:
| Value | Explanation |
| value | The default value of the attribute |
| #REQUIRED | The attribute is required |
| #IMPLIED | The attribute is not required |
| #FIXED value | The attribute value is fixed |
Default attribute value
<!ELEMENT square EMPTY>
<!ATTLIST square width CDATA "0">
In the example above, the "square" element is defined to be an empty element with a "width" attribute of type CDATA. If no width is specified, it has a default value of 0.
Enumerated values
<!ATTLIST payment type (check|cash) "cash">
Example:
<payment type="check" />
or
<payment type="cash" />
Why is elements better then attributes
Data can be stored in child elements or in attributes.Attribute:
Element:
Some of the problems with attributes are:
- 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
Example:
Entities
Entities are variables used to define shortcuts to standard text or special characters.- Entities can be declared internal or external
<!ENTITY entity-name "entity-value">
Example
<!ENTITY copyright "Copyright W3Schools.">
External
<!ENTITY entity-name SYSTEM "URI/URL">
Example
<!ENTITY writer SYSTEM "http://www.w3schools.com/entities.dtd">
