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XSD Attributes

All attributes are declared as simple types.

Only complex elements can have attributes!

What is an Attribute?

Simple elements cannot have attributes. If an element has attributes, it is considered to be of complex type. But the attribute itself is always declared as a simple type. This means that an element with attributes always has a complex type definition.

How to Define an Attribute

The syntax for defining an attribute is:

<xs:attribute name="xxx" type="yyy"/>

Here are an XML element with an attribute:

<lastname lang="EN">Smith</lastname>

And here are a corresponding simple attribute definition:

<xs:attribute name="lang" type="xs:string"/>

Common XML Schema Data Types

XML Schema has a lot of built-in data types. Here is a list of the most common types:

• xs:string
• xs:decimal
• xs:integer
• xs:boolean
• xs:date
• xs:time

Declare Default and Fixed Values for Attributes

Attributes can have a default value OR a fixed value specified.

A default value is automatically assigned to the attribute when no other value is specified. In the following example the default value is "EN":

<xs:attribute name="lang" type="xs:string" default="EN"/>

A fixed value is also automatically assigned to the attribute. You cannot specify another value. In the following example the fixed value is "EN":

<xs:attribute name="lang" type="xs:string" fixed="EN"/>

Creating Optional and Required Attributes

All attributes are optional by default. To explicitly specify that the attribute is optional, use the "use" attribute:

<xs:attribute name="lang" type="xs:string" use="optional"/>

To make an attribute required:

<xs:attribute name="lang" type="xs:string" use="required"/>

Restrictions on Content

When an XML element or attribute has a type defined, it puts a restriction for the element's or attribute's content. If an XML element is of type "xs:date" and contains a string like "Hello Mother", the element will not validate.

But, there is more... with XML Schemas, you can add your own restrictions to your XML elements and attributes. These restrictions are called facets. You can read more about facets in the next chapter.

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