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XML Schema unique Element

Definition and Usage

The unique element defines that an element or an attribute value must be unique within the scope.

The unique 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)

<unique
id=ID
name=NCName
any attributes
>

(annotation?,(selector,field+))

</unique>

(The ? sign declares that the element can occur zero or one time inside the unique element)

Attribute	Description
id	Optional. Specifies a unique ID for the element
name	Required. Specifies a name for the element
any attributes	Optional. Specifies any other attributes with non-schema namespace