XML Schema
Define XML structure with powerful data types and constraints
📐 What is XML Schema?
XML Schema (XSD) is a more powerful alternative to DTD for defining XML structure. It supports data types, namespaces, and complex validation rules, making it ideal for modern XML applications.
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="note" type="xs:string"/>
</xs:schema>
Key Schema Concepts
Data Types
Built-in type validation
type="xs:integer"Complex Types
Define element structures
<xs:complexType>Restrictions
Constrain values
<xs:minInclusive>Namespaces
Avoid naming conflicts
xmlns:xs="..."🔹 Basic XML Schema
XML Schema documents are written in XML and define the structure of other XML documents. They specify elements, their data types, and validation rules. Schemas are more powerful than DTDs and support modern XML features like namespaces.
Schema File (note.xsd):
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>XML Document:
<?xml version="1.0"?>
<note xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="note.xsd">
<to>Alice</to>
<from>Bob</from>
<heading>Reminder</heading>
<body>Don't forget the meeting!</body>
</note>🔹 Simple Types
Simple types define elements that contain only text with no child elements or attributes. XML Schema provides many built-in types like string, integer, date, and boolean. You can also create custom simple types with restrictions.
<!-- Built-in simple types -->
<xs:element name="name" type="xs:string"/>
<xs:element name="age" type="xs:integer"/>
<xs:element name="price" type="xs:decimal"/>
<xs:element name="birthdate" type="xs:date"/>
<xs:element name="active" type="xs:boolean"/>
<!-- Custom simple type with restriction -->
<xs:simpleType name="ageType">
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
<xs:maxInclusive value="120"/>
</xs:restriction>
</xs:simpleType>
<xs:element name="age" type="ageType"/>Common Built-in Types:
xs:string - Text data
xs:integer - Whole numbers
xs:decimal - Decimal numbers
xs:date - Date (YYYY-MM-DD)
xs:boolean - true/false
🔹 Complex Types
Complex types define elements that contain child elements or attributes. They allow you to create structured data with nested elements. Use sequence for ordered elements, choice for alternatives, and all for unordered elements.
<!-- Complex type with sequence -->
<xs:complexType name="bookType">
<xs:sequence>
<xs:element name="title" type="xs:string"/>
<xs:element name="author" type="xs:string"/>
<xs:element name="price" type="xs:decimal"/>
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required"/>
</xs:complexType>
<xs:element name="book" type="bookType"/>
<!-- Complex type with choice -->
<xs:complexType name="contactType">
<xs:choice>
<xs:element name="email" type="xs:string"/>
<xs:element name="phone" type="xs:string"/>
</xs:choice>
</xs:complexType>🔹 Restrictions and Facets
Restrictions allow you to constrain values using facets. Facets define specific rules like minimum/maximum values, string length, patterns, and enumerated values. This provides powerful validation beyond simple data types.
<!-- String length restriction -->
<xs:simpleType name="usernameType">
<xs:restriction base="xs:string">
<xs:minLength value="3"/>
<xs:maxLength value="20"/>
</xs:restriction>
</xs:simpleType>
<!-- Pattern restriction (regex) -->
<xs:simpleType name="emailType">
<xs:restriction base="xs:string">
<xs:pattern value="[^@]+@[^@]+\.[^@]+"/>
</xs:restriction>
</xs:simpleType>
<!-- Enumeration -->
<xs:simpleType name="categoryType">
<xs:restriction base="xs:string">
<xs:enumeration value="fiction"/>
<xs:enumeration value="non-fiction"/>
<xs:enumeration value="reference"/>
</xs:restriction>
</xs:simpleType>
<!-- Numeric range -->
<xs:simpleType name="percentType">
<xs:restriction base="xs:integer">
<xs:minInclusive value="0"/>
<xs:maxInclusive value="100"/>
</xs:restriction>
</xs:simpleType>🔹 Occurrence Indicators
Occurrence indicators control how many times an element can appear. Use minOccurs and maxOccurs to specify exact counts, ranges, or unlimited occurrences. This provides flexible validation for repeating elements.
<xs:element name="library">
<xs:complexType>
<xs:sequence>
<!-- Exactly one -->
<xs:element name="name" type="xs:string"/>
<!-- Optional (0 or 1) -->
<xs:element name="description" type="xs:string"
minOccurs="0" maxOccurs="1"/>
<!-- One or more -->
<xs:element name="book" type="bookType"
minOccurs="1" maxOccurs="unbounded"/>
<!-- Zero or more -->
<xs:element name="review" type="xs:string"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>🔹 Complete Schema Example
This comprehensive example demonstrates a complete XML Schema for a bookstore catalog. It includes simple types, complex types, attributes, restrictions, and occurrence indicators working together to create a robust validation schema.
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<!-- Simple type definitions -->
<xs:simpleType name="isbnType">
<xs:restriction base="xs:string">
<xs:pattern value="[0-9]{3}-[0-9]{10}"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="categoryType">
<xs:restriction base="xs:string">
<xs:enumeration value="fiction"/>
<xs:enumeration value="non-fiction"/>
<xs:enumeration value="reference"/>
</xs:restriction>
</xs:simpleType>
<!-- Complex type for book -->
<xs:complexType name="bookType">
<xs:sequence>
<xs:element name="title" type="xs:string"/>
<xs:element name="author" type="xs:string"
minOccurs="1" maxOccurs="unbounded"/>
<xs:element name="isbn" type="isbnType"/>
<xs:element name="price" type="xs:decimal"/>
<xs:element name="description" type="xs:string"
minOccurs="0"/>
</xs:sequence>
<xs:attribute name="id" type="xs:ID" use="required"/>
<xs:attribute name="category" type="categoryType"
default="fiction"/>
</xs:complexType>
<!-- Root element -->
<xs:element name="bookstore">
<xs:complexType>
<xs:sequence>
<xs:element name="book" type="bookType"
minOccurs="1" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>