XML Schema Attributes

Attributes provide additional information about elements. In an XML Schema, you declare attributes using xs:attribute. XSD attribute declarations give you control over the attribute name, data type, whether it is required, and what default or fixed values it can hold — all far more precisely than DTD allows.

Basic Attribute Declaration Syntax

<xs:attribute name="attribute-name" type="xs:datatype"/>

Simple Example

<xs:attribute name="language" type="xs:string"/>

This declares an optional attribute named language that holds a text string.

use Attribute — Required, Optional, or Prohibited

The use attribute on xs:attribute controls whether the XML attribute is required, optional, or not allowed.

<xs:attribute name="productID" type="xs:string" use="required"/>
<xs:attribute name="discount"  type="xs:decimal" use="optional"/>
<xs:attribute name="internalCode" type="xs:string" use="prohibited"/>

default and fixed Values

default

When the attribute is absent, the parser substitutes the default value. The element must be valid even without the attribute.

<xs:attribute name="currency" type="xs:string" default="INR"/>

fixed

The attribute must have exactly this value, or it must be absent. Any other value fails validation.

<xs:attribute name="country" type="xs:string" fixed="India"/>

Declaring Attributes Inside a Complex Type

Attributes always belong to a complex type. They are declared after the content model (xs:sequence, xs:all, etc.) but before the closing </xs:complexType> tag.

<xs:complexType name="ProductType">
  <xs:sequence>
    <xs:element name="name"  type="xs:string"/>
    <xs:element name="price" type="xs:decimal"/>
  </xs:sequence>
  <xs:attribute name="id"       type="xs:ID"      use="required"/>
  <xs:attribute name="category" type="xs:string"  use="optional"/>
  <xs:attribute name="currency" type="xs:string"  default="INR"/>
</xs:complexType>

Valid XML for this type

<product id="P001" category="electronics">
  <name>Tablet 10"</name>
  <price>14999</price>
</product>

xs:ID — Unique Attribute

xs:ID ensures the attribute value is unique across the entire document. It behaves like a primary key in a database table.

<xs:attribute name="empID" type="xs:ID" use="required"/>

Valid XML

<employee empID="E001">Asha</employee>
<employee empID="E002">Biju</employee>

Invalid XML — duplicate ID

<employee empID="E001">Asha</employee>
<employee empID="E001">Biju</employee>

xs:IDREF — Linking Elements

xs:IDREF means the attribute value must match an xs:ID value that exists somewhere in the document. This creates links between elements.

<xs:attribute name="managerRef" type="xs:IDREF" use="optional"/>

Example XML

<employees>
  <employee empID="E001">Asha</employee>
  <employee empID="E002" managerRef="E001">Biju</employee>
</employees>

Biju's managerRef points to Asha's empID. The validator checks that E001 exists in the document.

Restricting Attribute Values

Use xs:simpleType with xs:restriction inside the attribute declaration to limit the allowed values.

Enumeration — restrict to a list of values

<xs:attribute name="status">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:enumeration value="active"/>
      <xs:enumeration value="inactive"/>
      <xs:enumeration value="suspended"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>

Pattern — restrict to a format

<xs:attribute name="pinCode">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:pattern value="[0-9]{6}"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>

This attribute accepts only exactly 6 digits — a valid Indian postal code format.

Global vs Local Attribute Declarations

Global Attribute

Declared at the schema root level and referenced using ref in multiple complex types.

<xs:attribute name="lang" type="xs:string" default="en"/>

<xs:complexType name="ArticleType">
  <xs:sequence>
    <xs:element name="title" type="xs:string"/>
  </xs:sequence>
  <xs:attribute ref="lang"/>
</xs:complexType>

Complete Example — Employee Directory Schema

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="directory">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="employee" minOccurs="1" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="name"       type="xs:string"/>
              <xs:element name="department" type="xs:string"/>
              <xs:element name="salary"     type="xs:decimal"/>
            </xs:sequence>
            <xs:attribute name="empID"  type="xs:ID"     use="required"/>
            <xs:attribute name="joinDate" type="xs:date" use="required"/>
            <xs:attribute name="status">
              <xs:simpleType>
                <xs:restriction base="xs:string">
                  <xs:enumeration value="active"/>
                  <xs:enumeration value="inactive"/>
                </xs:restriction>
              </xs:simpleType>
            </xs:attribute>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Key Points to Remember

• xs:attribute declares an attribute in an XML Schema.
• use="required" forces the attribute to appear; use="optional" makes it optional.
• default provides a fallback value when the attribute is absent.
• fixed locks the attribute to one specific value permanently.
• xs:ID ensures the attribute value is unique across the document.
• xs:IDREF creates a cross-reference to an xs:ID value elsewhere in the document.
• Restrictions inside xs:simpleType limit attributes to enumerated values or patterns.