xsIDREF

xsIDREF, commonly written as xs:IDREF in XML Schema, is a built-in datatype used to reference the value of an attribute that is declared as xs:ID within the same XML document. The purpose is to model a reference or link from one element to another identified element.

Semantics and validation: The lexical space of xs:IDREF is the set of NCName strings. During validation, the value of an attribute of type xs:IDREF must match the value of some attribute annotated as xs:ID in the instance document. IDs (xs:ID) must be unique across the entire document, and each IDREF must refer to an existing ID. If no matching ID is found, a validation error is produced. This mechanism enables simple, document-local referencing without introducing external identifiers.

Usage: In a typical scenario, one element defines an identifier with type xs:ID, and other elements reference that identifier using an attribute of type xs:IDREF. This is common in data models that represent graphs, hierarchies, or linked records where elements need to point to other elements by their IDs.

Relation to related types: xs:IDREF refers to a single target ID. For multiple references, the type xs:IDREFS can be used, which is a whitespace-separated list of IDs. xs:ID defines identity constraints, whereas xs:IDREF defines a reference constraint; there is no cross-document resolution in standard XML Schema 1.0.

See also: xs:ID, xs:IDREFS, identity constraints.