XML : eXtensible Markup Language :: Introduction to XML Schema

• Introduction to Schema
• Need for Schema
• Benefits of XML Schema
• Limitations of DTDs
• Schema Data Types
• Schema Elements
• Download Examples

Dictionary Meaning:

A diagrammatic representation; an outline or a model.

In Software Context :

A model used to describe the structure of a database. Defines internal structures such as tables, fields and the relationship between them.

In XML Context :

A set of rules to constrain the structure and articulate the information set of XML documents.

• XML schema is a technology created by Microsoft that implements a subset of a W3C note
• Improves upon many weaknesses of DTDs such as the cryptic syntax, the lack of data typing, the closed content model, and the lack of support for namespaces
• Supports a wide range of data types such as integers, floating point numbers, dates and times
• Includes support for other features, such as an open content model and namespace integration

• Created using XML syntax, so, the processing of XML Schemas and XML Instances by the same software tool is possible.
• Less expenditure for the data management and data administration results in overall low programming expenditure.
• As XML Schemas were finalized after the Namespace Recommendation, the concept of namespaces was available to be included in the design. Thus, XML Schemas enables to define vocabularies that utilize namespace declarations.
• XML Schemas allow you to validate text element content based on built-in and user-defined data types.
• XML Schemas allows to easily create complex and reusable content models.
• XML Schemas allows to model concepts such as object inheritance and type substitution.
• XML Schemas are Extensible
  • Reuse your Schema in other Schemas
  • Create your own data types derived from the standard types
  • Reference multiple schemas in the same document

Limitations of DTDs

• DTDs are written in a non-XML syntax
• DTDs are not extensible
• Do not support namespaces
• Only offer extremely limited datatyping

Schemas overcome these limitations and allow Web applications to exchange XML data more robustly, without relying on adhoc validation tools.

• XML Schemas are extensible to future additions
• XML Schemas are richer and more powerful than DTDs
• XML Schemas are written in XML
• XML Schemas support data types
• XML Schemas support namespaces
• XML Schema is a W3C Standard
• XML Schema became a W3C Recommendation on 02. May 2001.

Additional data types are generated from the basic built-in data types, which are called basetype data types

The <schema> Element

The <schema> element is the root element for every XML Schema:

<?xml version="1.0"?>
<xs:schema>
...
...
</xs:schema>

Example

Attribute

Simple elements cannot have attributes. If an element has attributes, it is considered to be of a complex type. But the attribute itself is always declared as a simple type

Syntax

<xsd:attribute name="aaa" type="bbb"/>

Here aaa is the name of the attribute and bbb specifies the data type of the attribute.

XML Schema has a lot of built-in data types. Some common types are:

• xsd:string
• xsd:decimal
• xsd:integer
• xsd:boolean
• xsd:date
• xsd:time

Example

<authorname lang="HND">Tom</authorname>
    here is the attribute definition:
<xs:attribute name="lang" type="xs:string"/>

Complex Type

Simple Type