How to Understand XSD

An XSD defines the structures that can occur in an XML tree.

An XSD is an XML Schema Definition, a set of rules describing the structure of XML markup. XSDs are used to define the structures that one or more XML documents must conform to. Because XML is customizable and adaptable to different uses, XSDs are a way for developers to create a set structure that they want to use within their XML data. XSDs reflect the nature of a set of data in a similar way to a Relational Database design. An XSD defines the various data elements that can be included in the relevant XML, and indicates the data-types and relationships within these elements.

Instructions

1. Look at sample XSDs to familiarize yourself with the syntax. The best way to gain an understanding of how XSDs work is to look at some real examples, together with examples of XML conforming to them. XSDs are also written in XML, and must comprise a well-formed tree structure. An XSD typically contains a list of the elements that are allowed in the XML; the attributes that these elements can have; the elements that are child elements of others; the number of child elements that can occur; the order of child elements; the data types and values for both elements and attributes.

2. Learn how XSDs represent elements. In the following example, the XSD specifies an element named "picture," which should consist of a text string:

   <xs:element name="picture" type="xs:string"/>

   An actual XML element reflecting this could be:

   <picture>imagename.jpg</picture>

   Here the picture element is given a default value:

   <xs:element name="picture" type="xs:string" default="imagename.jpg"/>

3. Learn how element attributes are represented within XSDs. Attributes are used where you have additional information about a data element that does not comprise a data element in itself. In this example, an attribute with the name "width" is specified as being an integer and required in all cases:

   <xs:attribute name="width" type="xs:integer" use="required"/>

   An example in XML would be:

   <picture width="300">imagename.jpg</picture>

   Note that although the attribute type is an integer, it must still be listed between quotation marks.

4. Learn how Complex Types are reflected in XSDs. Complex Types are mostly elements that can contain child elements. In this example, the "picture" element is defined as a child of another element named "gallery" along with a further element named "title," which is also a text string:

   <xs:element name="gallery">

   <xs:complexType>

   <xs:sequence>

   <xs:element name="title" type="xs:string"/>

   <xs:element name="picture" type="xs:string" default="imagename.jpg"/>

   </xs:sequence>

   </xs:complexType>

   </xs:element>

   Sample XML reflecting this structure could be as follows:

   <gallery>

   <title>Lovely Pictures</title>

   <picture width="300">picture1.jpg</picture>

   <picture width="250">picture2.jpg</picture>

   </gallery>

5. Learn how XSDs indicate the number of times that an element can occur. In this excerpt, the "picture" element is specified as being able to occur any number of times:

   <xs:element name="picture" type="xs:string" default="imagename.jpg" maxOccurs="unbounded"/>

   This example states that the "title" element is optional as the minimum number of occurrences is set to zero:

   <xs:element name="title" type="xs:string" minOccurs="0"/>

Tips & Warnings

• If your XSD is long and complex, it helps to split it into sections and use comments to make it a little more readable.

• Often, XSDs are generated from XML, but you should always use your XSDs to validate that your XML documents are correctly structured.

References

• W3Schools: XML Schema Example
• W3Schools: XML Schema Simple Elements
• informIT: Creating Attributes
• W3Schools: XML Schema Complex Elements

Resources

• Webucator: XML Schema Tutorial
• O'Reilly Media: Using W3C XML Schema