XML Schema

Name

xs:unique — Definition of a uniqueness constraint.

Synopsis

<xs:unique
           id            = xs:ID
           name          = xs:NCName
           {any attributes with non-schema namespace}
           >
           Content: ((xs:annotation?), (xs:selector, xs:field+))
</xs:unique>

May be included in: xs:element (within xs:all), xs:element (reference or local definition), xs:element (global definition)

Description

xs:unique is used to define simple or compound constraints, which unambiguously identify each element in which they are present, from a selected list of subelements within the scope of a root element.

xs:unique is very similar to xs:key. Like the constraints defined with xs:key, constraints defined with xs:unique are unique in the scope of their root element and may be referenced by xs:keyref. The only difference between xs:unique and xs:key is that the xs:unique keys may be undefined in any of the elements in the selection list, while xs:key identifiers must be defined for all the elements in the selection list.

The root element for the constraint is the element in which the constraint is defined. The location of the root element must be carefully chosen since the unity of the constraint is checked only within the node elements that are its children. Defining a root element that has multiple occurrences within a document leaves the possibility of defining local constraints that are unique only in the scope of each occurrence of a document. When a constraint is global to a document, defining it using the document element as a root may ...