# XML Schema unique Element

Complete XML Schema Reference

## 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)

### Element Information

- Parent elements: element

### Syntax

<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

Complete XML Schema Reference

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