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# XML Schema key Element * * Complete XML Schema Reference Manual](#) * * * ## Definition and Usage The key element specifies that an attribute or element value (or set of values) must be a key within the specified scope. The scope of the key is the containing element in the instance document. A key means the data should be unique, non-null, and always present within the specified scope. The key element must contain the following elements in order: * The selector element contains an XPath expression that specifies a set of elements within which the values specified by the field must be unique. ### Element Information * **Parent Element:** element ### Syntax (annotation?,(selector,field+)) (The ? symbol declares that the element may appear zero or one time within the key element, 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. | | any attributes | Optional. Specifies any other attributes with non-schema namespaces. | * * Complete XML Schema Reference Manual](#)