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XML Schema key Element

Definition and Usage

The key element specifies an attribute or element value as a key (unique, non-nullable, and always present) within the containing element in an instance document.

The key 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

<key
id=ID
name=NCName
any attributes
>

(annotation?,(selector,field+))

</key>

(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 key element)

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

Complete XML Schema Reference