XML Schema Complex Types

A simple type holds a single value — a number, a string, a date. A complex type is richer: it can contain child elements, carry attributes, or both. Any element that contains other elements or has attributes must be declared as a complex type in XML Schema.

Defining a Complex Type

<xs:complexType name="TypeName">
  <!-- content model (xs:sequence, xs:all, or xs:choice) -->
  <!-- attributes -->
</xs:complexType>

Content Models Inside Complex Types

1. xs:sequence — Children in Order

All child elements must appear in the declared order. This is the most common content model.

<xs:complexType name="AddressType">
  <xs:sequence>
    <xs:element name="street" type="xs:string"/>
    <xs:element name="city"   type="xs:string"/>
    <xs:element name="state"  type="xs:string"/>
    <xs:element name="pinCode" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

Valid XML

<address>
  <street>12, MG Road</street>
  <city>Bangalore</city>
  <state>Karnataka</state>
  <pinCode>560001</pinCode>
</address>

2. xs:all — Children in Any Order

All declared child elements must appear, but in any order. Each can appear at most once.

<xs:complexType name="PersonType">
  <xs:all>
    <xs:element name="name"  type="xs:string"/>
    <xs:element name="email" type="xs:string"/>
    <xs:element name="phone" type="xs:string"/>
  </xs:all>
</xs:complexType>

Valid XML — children in different orders

<person>
  <phone>9876543210</phone>
  <name>Sunita Verma</name>
  <email>sunita@mail.com</email>
</person>

3. xs:choice — Only One Child Allowed

The element must contain exactly one of the listed child elements.

<xs:complexType name="PaymentType">
  <xs:choice>
    <xs:element name="creditCard" type="xs:string"/>
    <xs:element name="upiID"      type="xs:string"/>
    <xs:element name="netBanking" type="xs:string"/>
  </xs:choice>
</xs:complexType>

Valid XML

<payment><upiID>user@paytm</upiID></payment>

Invalid XML — two choices

<payment>
  <upiID>user@paytm</upiID>
  <creditCard>4111...</creditCard>
</payment>

Complex Types with Attributes

Attributes are declared inside the complex type, after the content model.

<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:complexType>

Complex Type with Text Content and Attributes

Sometimes an element carries both a text value and attributes. Use xs:simpleContent with xs:extension for this.

<xs:complexType name="PriceType">
  <xs:simpleContent>
    <xs:extension base="xs:decimal">
      <xs:attribute name="currency" type="xs:string" default="INR"/>
    </xs:extension>
  </xs:simpleContent>
</xs:complexType>

Valid XML

<price currency="USD">29.99</price>
<price>1499</price>   <!-- currency defaults to INR -->

Anonymous vs Named Complex Types

Named Complex Type

Defined separately and reused by multiple elements using the type attribute.

<xs:complexType name="ContactType">
  <xs:sequence>
    <xs:element name="phone" type="xs:string"/>
    <xs:element name="email" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

<xs:element name="customerContact" type="ContactType"/>
<xs:element name="supplierContact" type="ContactType"/>

Anonymous Complex Type

Defined directly inside an element declaration. Used only once.

<xs:element name="invoice">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="invoiceNo" type="xs:string"/>
      <xs:element name="amount"    type="xs:decimal"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

Extending Complex Types

A complex type can extend another — like inheritance in programming. The child type gets all parent elements plus new ones.

<xs:complexType name="PersonType">
  <xs:sequence>
    <xs:element name="name"  type="xs:string"/>
    <xs:element name="email" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="EmployeeType">
  <xs:complexContent>
    <xs:extension base="PersonType">
      <xs:sequence>
        <xs:element name="employeeID" type="xs:string"/>
        <xs:element name="department" type="xs:string"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

EmployeeType includes name, email (from PersonType) plus employeeID and department.

Valid XML for EmployeeType

<employee>
  <name>Rajesh Kumar</name>
  <email>rajesh@company.com</email>
  <employeeID>EMP-2024-101</employeeID>
  <department>Engineering</department>
</employee>

Content Model Comparison

Key Points to Remember

• Complex types define elements that contain child elements or attributes.
• xs:sequence requires children in a specific order.
• xs:all allows children in any order but each appears at most once.
• xs:choice allows only one of the listed children.
• Use xs:simpleContent when an element has both a text value and attributes.
• Named complex types are reusable; anonymous types are used only once.
• xs:extension lets one complex type inherit and extend another.