W3C XML Schema: A Brief Overview

The schema language supported directly by the XQuery recommendation is W3C XML Schema, which is a W3C Recommendation with two normative parts:[*]

Part 1 Structures

Defines a language for defining schemas, which express constraints on XML documents.

Part 2 Datatypes

Defines a rich set of built-in types that can be applied to XML documents through schemas, as well as through other mechanisms.

The XML Schema built-in types defined in Part 2 are the basis for the atomic types used in XQuery. Regardless of whether schemas are present, these types can be used in queries to represent common datatypes such as strings, numbers, dates, and times. All XQuery implementations support this basic set of types.

XQuery also interacts with Part 1 of XML Schema. Schema definitions can be used in queries to validate input documents, intermediate values, and result elements.

XQuery does not currently provide specific support for other schema languages or validation mechanisms such as DTDs, RELAX NG, or Schematron. However, a processor can be written that validates a document using one of these schema languages and annotates its elements and attributes with appropriate types.

Element and Attribute Declarations

Elements and attributes are the most basic components of an XML document. The catalog schema has a declaration for each of the different kinds of elements and attributes in the catalog, such as product, number, and dept. Elements are declared in the schema ...

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