Real World XML

Specifying Attribute Constraints and Defaults

As with elements, you can specify the type of attributes, but unlike elements, attributes must be of a simple type. And you don't use minOccurs and maxOccurs for attributes because attributes can appear only once, at most. Instead, you use a different syntax when constraining attributes.

You declare attributes with the <xsd:attribute> element, and the <xsd:attribute> element itself has a type attribute that gives the attribute's (simple) type. So how do you indicate whether an attribute is required or optional, or whether there's a default value, or even whether the value of the attribute is fixed at a certain value? You use the <xsd:attribute> element's use and value attributes.

The use attribute ...