xsd: How to extend a type with an unordered list of elements - xsd

Question

This is a part of my xml schema
<xs:complexType name="Friend">
<xs:all>
<xs:element name="name" type="xs:string" />
<xs:element name="phone" type="xs:string" />
<xs:element name="address" type="xs:string" />
</xs:all>
</xs:complexType>
<xs:complexType name="Coworker">
<xs:all>
<xs:element name="name" type="xs:string" />
<xs:element name="phone" type="xs:string" />
<xs:element name="office" type="xs:string" />
</xs:all>
</xs:complexType>
For better maintainability, I would like to have the shared attributes in an (abstract) super type or something like that. But more important, I want that all elements are unordered and also optional.
Is this possible, and what is the best way to do it?

Answer 1

You have to limit yourself a little bit, some of the things you are trying to do are not possible in XML Schema.
Suppose you introduce a complex type called Person to be a super-type of Friend and Coworker. Here are your options:
Replace xs:all with xs:sequence, remove name and phone from the sub-types, add to the super-type, and add inheritance. Your elements now have to be ordered, but you can make them individually optional. It is illegal to use xs:all in type hierarchies in XML Schema, because the processor cannot tell where the parent content model stops and the child content model starts.
Replace xs:all with <xs:choice maxOccurs="unbounded"> in both types, and add your inheritance. Then your elements become unordered again, but they may repeat.
So in conclusion: given your type names up there, I would guess that your requirements will not be exactly met. I would go for the first option: insisting on arbitrary element order is often not as useful as it seems.

Answer 2

One-and-half year after this question and the accepted answer were posted, XSD 1.1 was published. In this version it is possible to specify what the OP asked for because a number of restriction on xs:all were lifted. One of them is that it is now possible to extend an xs:all.
Using XSD 1.1 you can specify the following:
<xs:complexType name="Person" abstract="true">
<xs:all>
<xs:element name="name" type="xs:string" minOccurs="0" />
<xs:element name="phone" type="xs:string" minOccurs="0" />
</xs:all>
</xs:complexType>
<xs:complexType name="Friend">
<xs:complexContent>
<xs:extension base="Person">
<xs:all>
<xs:element name="address" type="xs:string" minOccurs="0" />
</xs:all>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<xs:complexType name="Coworker">
<xs:complexContent>
<xs:extension base="Person">
<xs:all>
<xs:element name="office" type="xs:string" minOccurs="0" />
</xs:all>
</xs:extension>
</xs:complexContent>
</xs:complexType>
This defines the following types:
Person: an abstract type with optional unordered name and phone elements;
Friend: extends Person adding an optional address element to the list of unordered elements;
Coworker: extends Coworker adding an optional office element to the list of unordered elements.
Note that this solution does not work for every XML processor: even though 8 years have passed since the publication of XSD 1.1, a lot of processors still only support XSD 1.0.