Identity constraints allow you to uniquely identify nodes in a document and ensure the integrity of references between them. This chapter explains how to define and use identity constraints.
There are three categories of identity constraints.
• Uniqueness constraints enforce that a value (or combination of values) is unique within a specified scope. For example, all product numbers must be unique within a catalog.
• Key constraints also enforce uniqueness, and additionally require that all values be present. For example, every product must have a number and it must be unique within a catalog.
• Key references enforce that a value (or combination of values) corresponds to a ...