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# XML Schema keyref Element * * Complete XML Schema Reference](#) * * * ## Definition and Usage The keyref element specifies that an attribute or element value (or set of values) must correspond to the values of a specified key or unique element. The keyref element must contain the following elements in order: * There must be one and only one selector element. * The selector element contains an XPath expression that specifies a set of elements where the values specified by field must be unique. ### Element Information * **Parent element:** element ### Syntax (annotation?,(selector,field+)) (The ? symbol declares that in the key element, the element may appear zero or one time, and the + symbol declares that the element must appear one or more times.) | Attribute | Description | | --- | --- | | id | Optional. Specifies a unique ID for the element. | | name | Required. Specifies the name of the key element. | | refer | Required. Specifies the name of a key or unique element defined in this schema (or in another schema indicated by the specified namespace). The refer value must be a qualified name (QName). The type may include a namespace prefix. | | any attributes | Optional. Specifies any other attributes with non-schema namespaces. | * * Complete XML Schema Reference](#)