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

(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)