XML Identity Constraints — <xsd:keyref> Element

The keyref element specifies that an attribute or element value correspond to those of the specified key or unique element.

The keyref element MUST contain the following (in order):

one and only one selector element (contains an XPath expression that specifies the set of elements across which the values specified by field must be unique)
one or more field elements (contains an XPath expression that specifies the values that must be unique for the set of elements specified by the selector element)

Parent Elements

element

Syntax

<key
id=ID
name=NCName
refer=QName
any attributes

>

(annotation?,(selector,field+))

</include>

Note: The ? sign declares that the element can occur zero or one time, and the + sign declares that the element must occur one or more times inside the keyref element.

Attributes

id: Optional. Specifies a unique ID for the element.
name: Required. Specifies the name of the keyref element.
ref: Required. Specifies the name of a key or unique element defined in this or another schema.
any attributes: Optional. Specifies any other attributes with non-schema namespace.

For complete information on this and all XML elements, see the W3C XML Schema Part 1: Structures Specification: http://www.w3.org/TR/xmlschema-1/.