Editing XML schema (section)

==== Advantages over DTDs ====

Features available in XSD that are missing from DTDs include:

* Names of elements and attributes are namespace-aware
* Constraints ("simple types") can be defined for the textual content of elements and attributes, for example to specify that they are numeric or contain dates. A wide repertoire of simple types are provided as standard, and additional user-defined types can be derived from these, for example by specifying ranges of values, regular expressions, or by enumerating the permitted values.
* Facilities for defining uniqueness constraints and referential integrity are more powerful: unlike the ID and IDREF constraints in DTDs, they can be scoped to any part of a document, can be of any data type, can apply to element as well as attribute content, and can be multi-part (for example the combination of first name and last name must be unique).
* Many requirements that are traditionally handled using parameter entities in DTDs have explicit support in XSD: examples include substitution groups, which allow a single name (such as "block" or "inline") to refer to a whole class of elements; complex types, which allow the same content model to be shared (or adapted by restriction or extension) by multiple elements; and model groups and attribute groups, which allow common parts of component models to be defined in one place and reused.
* XSD 1.1 adds the ability to define arbitrary assertions (using XPath expressions) as constraints on element content.

XSD schemas are conventionally written as XML documents, so familiar editing and transformation tools can be used.

As well as validation, XSD allows XML instances to be annotated with type information (the [[PSVI|Post-Schema-Validation Infoset (PSVI)]]) which is designed to make manipulation of the XML instance easier in application programs. This may be by mapping the XSD-defined types to types in a programming language such as Java ("data binding") or by enriching the type system of XML processing languages such as XSLT and XQuery (known as "schema-awareness").