XML schema for elements with same name but different sub-structure depending on context - xsd

Question

I try to define a schema for XML documents I receive.
The documents look like:
<root>
<items>
<group name="G-1">
<item name="I-1"/>
<item name="I-2"/>
<item name="I-3"/>
<item name="I-4"/>
</group>
</items>
<data>
<group name="G-1" place="here">
<customer name="C-1">
<item name="I-1" count="3"/>
<item name="I-2" count="4"/>
</customer>
<customer name="C-2">
<item name="I-3" count="7"/>
</customer>
</group>
</data>
</root>
I tried XmlSpy and xsd.exe from .NET 2.0. Both created schema definitions which allow below <group> any number of <item> and <customer> elements. But what I'm looking for should restrict <group> below <items> to <item> elements, and <group> below <data> to <customer> elements.
Is this something xml schema is not capable at all?

Answer 1

The key points (see XML Schema Runtime Polymorphism via xsi:type and Abstract Types for complete and correct context/placement/usage) are:
Create a base type with (abstract="true" to prevent it from being used directly)
Note: the ref attribute replaces the name attribute for elements defined elsewhere
<xs:complexType name="CustomerType" abstract="true" >
<xs:sequence>
<xs:element ref="cust:FirstName" />
<xs:element ref="cust:LastName" />
<xs:element ref="cust:PhoneNumber" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="customerID" type="xs:integer" />
</xs:complexType>
Create two or more derived types by extending or restricting the base type
<xs:complexType name="MandatoryPhoneCustomerType" >
<xs:complexContent>
<xs:restriction base="cust:CustomerType">
<xs:sequence>
<xs:element ref="cust:FirstName" />
<xs:element ref="cust:LastName" />
<xs:element ref="cust:PhoneNumber" minOccurs="1" />
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
and
<xs:complexType name="AddressableCustomerType" >
<xs:complexContent>
<xs:extension base="cust:CustomerType">
<xs:sequence>
<xs:element ref="cust:Address" />
<xs:element ref="cust:City" />
<xs:element ref="cust:State" />
<xs:element ref="cust:Zip" />
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
Reference the base type in an element
<xs:element name="Customer" type="cust:CustomerType" />
In your instance XML document, specify the specific derived type as an xsi:type attribute
<cust:Customer customerID="12345" xsi:type="cust:MandatoryPhoneCustomerType" >
<cust:FirstName>Dare</cust:FirstName>
<cust:LastName>Obasanjo</cust:LastName>
<cust:PhoneNumber>425-555-1234</cust:PhoneNumber>
</cust:Customer>
or:
<cust:Customer customerID="67890" xsi:type="cust:AddressableCustomerType" >
<cust:FirstName>John</cust:FirstName>
<cust:LastName>Smith</cust:LastName>
<cust:Address>2001</cust:Address>
<cust:City>Redmond</cust:City>
<cust:State>WA</cust:State>
<cust:Zip>98052</cust:Zip>
</cust:Customer>

Answer 2

Yes, XSD can handle this. I generated this schema from Visual Studio 2008 (much faster than doing it by hand) and it will do what you're looking for:
<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="root">
<xs:complexType>
<xs:sequence>
<xs:element name="items">
<xs:complexType>
<xs:sequence>
<xs:element name="group">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" name="item">
<xs:complexType>
<xs:attribute name="name" type="xs:string" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="name" type="xs:string" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="data">
<xs:complexType>
<xs:sequence>
<xs:element name="group">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" name="customer">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" name="item">
<xs:complexType>
<xs:attribute name="name" type="xs:string" use="required" />
<xs:attribute name="count" type="xs:unsignedByte" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="name" type="xs:string" use="optional" />
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="name" type="xs:string" use="required" />
<xs:attribute name="place" type="xs:string" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>