Summary of XML Schema Definition (XSD) Guide

## Introduction XML Schema Definition (XSD) is a formal language for describing the structure and data types of XML documents. It enables precise XML validation — not just syntactic (well-formedness), but also content-based (types, number of elements, order, attribute values, etc.). This material summarizes the fundamental concepts of XSD, design patterns, and practical examples. > Definition: An XML Schema (XSD) is an XML document (a file with the .xsd extension) that describes the allowed structure, data types, and constraints for XML instances. ## Basic Concepts ### Why Use XSD - Define the data format and rules for creating XML documents - Automatic validation before further processing - Specify precise data types (e.g., date, number) - Define required/optional elements and their occurrence ### Well-formed vs Valid - Well-formed: An XML document follows XML syntax (correct tag closing, escaping, etc.) - Valid: An XML document also conforms to the defined XSD (structure and data types) ## XSD File Structure - Root element: <xs:schema> - Namespace declaration: xmlns:xs="http://www.w3.org/2001/XMLSchema" - Optional: targetNamespace, elementFormDefault (qualified/unqualified) > Definition: elementFormDefault = "qualified" means that local elements must be namespace-qualified in the instance. ## Content Types ### xs:simpleType - Contains only text (no child elements) and may have restrictions (enumeration, pattern, length, minLength, maxLength, fractionDigits, totalDigits, whiteSpace) - Used for both elements and attributes Example of a simple element: ```xml <xs:element name="age" type="xs:integer"/> ``` ### xs:complexType - Elements with child elements, attributes, or both - Content models: <xs:sequence>, <xs:choice>, <xs:all> Example: ```xml <xs:element name="person"> <xs:complexType> <xs:sequence> <xs:element name="firstname" type="xs:string"/> <xs:element name="lastname" type="xs:string"/> </xs:sequence> <xs:attribute name="id" type="xs:integer" use="required"/> </xs:complexType> </xs:element> ## Order and Repetition of Elements - <xs:sequence>: exact order - <xs:choice>: exactly one of the listed elements - <xs:all>: elements can be in any order, each 0 or 1 time Occurrence Parameters: minOccurs, maxOccurs (e.g., maxOccurs="unbounded") Occurrence Example: ```xml <xs:element name="owner" type="xs:string" minOccurs="1" maxOccurs="unbounded"/> ## Attributes - Declaration: <xs:attribute name="att_name" type="xs:string" use="optional|required" default="..." fixed="..."/> - default: the parser supplies the value if the attribute is missing - fixed: the value is fixed and cannot be changed ## Value Restrictions and Constraints - enumeration, pattern (regex), length, minLength, maxLength, fractionDigits, totalDigits, minInclusive, maxInclusive, minExclusive, maxExclusive, whiteSpace ### Example of Enumeration and Pattern: ```xml <xs:element name="color"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="red"/> <xs:enumeration value="green"/> <xs:enumeration value="blue"/> </xs:restriction> </xs:simpleType> </xs:element> <xs:element name="plate"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:pattern value="[A-Z0-9]{7}"/> </xs:restriction> </xs:simpleType> </xs:element> ``` > Definition: `<xs:pattern>` uses Unicode regular expressions (Level 1). Patterns must not span multiple lines. ## Namespaces and References - To link an XSD to an XML instance, the `xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"` attribute is used, along with `xsi:schemaLocation="namespace path/to/schema.xsd"` or `xsi:noNamespaceSchemaLocation` when no targetNamespace is specified. - `elementFormDefault` and `targetNamespace` determine which elements are namespace-qualified. Example of linking: ```xml <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.example.com/persons persons.x