XML Schema

Name

xs:keyref — Definition of a key reference.

Synopsis

<xs:keyref
           id            = xs:ID
           name          = xs:NCName
           refer         = xs:QName
           {any attributes with non-schema namespace}
           >
           Content: ((xs:annotation?), (xs:selector, xs:field+))
</xs:keyref>

May be included in: xs:element (within xs:all), xs:element (reference or local definition), xs:element (global definition)

Description

xs:keyref is used to define a reference to a simple or compound key or to a unique constraint.

The syntax of xs:keyref is very similar to the syntax of xs:key or xs:unique. The only difference is the refer attribute, which must contain the name of the referred key or unique constraint.

The root element for the keyref is the element in which the key is defined. The key reference is often defined with the same root element as its referred key or unique constraint, in which case the matching is simple and straightforward.

The root element of the keyref may also be defined in any element for which it is in the scope of a matching constraint node table, i.e., in all the ancestors of the root element of the referred key or unique constraint. In this case, an additional check is performed; for each matching key reference, the validator should test that the key or unique value is defined only once in all the matching key or unique root elements.

Restrictions

Key references defined in an ancestor of their referenced key or unique constraint actually add additional constraints on the referenced key or the unique key, which may be unexpected. ...