DTD and Schema

Here is the explanation of DTD and Schema:

DTD:

DTD, which stands for Document Type Definition, is a formal specification used to define the structure and the legal elements and attributes of an XML document.

• It serves as a schema definition for XML, providing a set of rules that describe the data content and organization within an XML document.

• DTDs are often used to validate the correctness of XML documents and to ensure that they conform to a specific structure.

<!DOCTYPE bookstore [
    <!ELEMENT bookstore (title, author, price)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT author (#PCDATA)>
    <!ELEMENT price (#PCDATA)>
]>

The DTD above is interpreted like this:

• <!DOCTYPE bookstore– Defines that the root element of the document is bookstore
• <!ELEMENT bookstore– Defines that the bookstore element must contain the elements: “title, author, price”
• <!ELEMENT title– Defines the title element to be of type “#PCDATA“
• <!ELEMENT author– Defines the author element to be of type “#PCDATA“
• <!ELEMENT price– Defines the price element to be of type “#PCDATA“

Types of DTD:

There are two types of DTDs:

• Internal DTD
• External DTD.

Internal DTD:

In an internal DTD, the DTD declarations are included directly within the XML document. The DTD declarations are placed inside the document’s <!DOCTYPE> declaration.

Example of an XML document with an internal DTD:

<!DOCTYPE bookstore [
    <!ELEMENT bookstore (book+)>
    <!ELEMENT book (title, author, price)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT author (#PCDATA)>
    <!ELEMENT price (#PCDATA)>
]>

<bookstore>
    <book>
        <title>Introduction to XML</title>
        <author>John Doe</author>
        <price>29.95</price>
    </book>
   
</bookstore>

In this example, the DTD declarations are enclosed within the <!DOCTYPE> declaration before the root element (<bookstore>).

External DTD:

In an external DTD, the DTD declarations are stored in a separate external file and linked to the XML document. The external DTD file is referenced in the <!DOCTYPE> declaration using the SYSTEM keyword.

Example of an XML document with an external DTD:

books.dtd:

<!ELEMENT bookstore (book+)>
<!ELEMENT book (title, author, price)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT price (#PCDATA)>

books.xml:

<!-- bookstore.xsd -->

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <!-- Define the elements -->
  <xs:element name="bookstore">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="title" type="xs:string"/>
              <xs:element name="author" type="xs:string"/>
              <xs:element name="price" type="xs:decimal"/>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Now, let’s use this XML Schema to validate an XML document: