XML Basics - XML Schema

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XML Schema is an XML-based alternative to DTD. An XML Schema describes the structure of an XML document. The XML Schema language is also referred to as XML Schema Definition (XSD). An XML Schema defines the following:

• Elements and attributes that can appear in a document.
  
• Child elements.
  
• The order and number of child elements.
  
• Whether an element is empty or can include text.
  
• Data types for elements and attributes.
  
• Default and fixed values for elements and attributes.

The schema element is the first element in an XML schema file. The prefix xsd: is used to indicate the XML schema namespace:

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

The main sub-elements in an XML schema document are:

1. element – declares an element.
2. attribute – declares an attribute.
3. complexType – defines elements that can contain other elements and attributes.
4. simpleType – defines elements that can not contain other elements or attributes. These elements are of basic types, including string, date, integer etc.

The xsd:annotation and xsd:documentation tags are used for providing a description of the document. Here is an example:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:annotation>
      <xsd:documentation xml:lang="en">
            Bookstore schema for book.xml
      </xsd:documentation>
  </xsd:annotation>
</xsd:schema>

 

In our XML example the element bookstore is a complexType element because it contains book sub-elements.

<xsd:element name="bookstore" type="bookstoreinfo"/>

<xsd:complexType name="bookstoreinfo">
      <xsd:sequence>
            <xsd:element name="book" type="bookinfo"/>
      </xsd:sequence>
</xsd:complexType>

Our book element is also a complexType element.

<xsd:compleType name="bookinfo">
      <xsd:sequence>
            <xsd:element name="title" type="titleinfo">
            <xsd:element name="author" type="xsd:string"/>
            <xsd:element name="year" type=" xsd:string"/>
            <xsd:element name="price" type=" xsd:string"/>
      </xsd:sequence>
</xsd:complexType>

The sub-elements author, year and price are of type xsd:string. The sub-element title is of complexType because it has an attribute named id. To define an attribute use the following: 

<xsd:attribute name="id" type="xsd:string" use="required"/>

Use value required means this attribute is mandatory. By default attributes are optional.

Since XSD uses XML it is also extensible, and does not require learning a new language.

To manipulate an XML document, we need an XML parser. The parser loads the document into a computer's memory and can be manipulated using the DOM. The DOM treats the XML document as a tree.

XML is basically about defining data. XML is also used for storing data and sharing data between applications. With XML you can define documents that are understood by different software and applications. But to make these documents understandable to humans, your need to represent them. You can represent XML data using Cascading Style Sheets. But XSL is the preferred solution for converting XML documents into HTML. It can be used on the client side but the best solution is to use XSL on the server side and make it visible to any browser.

XML usage increases every day. With XML, financial information can be exchanged over the Internet easily. We should expect to see a lot about XML and B2B (Business To Business) in the near future. XML is going to be the main language for exchanging financial information between businesses over the Internet. So why wait? Let’s learn more about XML and start using it today!

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