A key constraint is just like a uniqueness constraint.

Except the selector and field specified must exist.

The other big difference is that keys can be referred to within the document.

A key is required to make a reference can be validated.

It is specified by the 'key' element with:

• a required 'name' attribute.

• One 'selector' child.

• One or more 'field' children.

A key declaration example:

<xs:key name="product-name">
<xs:selector xpath="."/>
<xs:field xpath="name"/>
</xs:key>