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Definitive XML Schema by Priscilla Walmsley

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Chapter 17 Identity constraints

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.

17.1 Identity constraint categories

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 ...

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