XML Schema for sequence of elements with same name but different attribute values?

Question

How can I specify an XML schema for an instance document like this:

Answer 1

Using Assertion Schema Component

Thanks to XML Schema 1.1, there is a way to enforce your requirement without having to import the xsi namespace and hack polymorphism into your XML document. XML Schema 1.1 brought two new components, assertions and type alternatives, providing the xs:assert and xs:alternative elements. They have a @test attribute in which conditions and constraints are being specified as XPath 2.0 expressions.

---

Even though xs:alternative is obviously meant to be the solution to your problem, I for my part have not yet succeeded in assigning alternative types based on an element's position. (The schema parser's understanding of node contexts seems to differ from the one I would expect based on the validated XML document structure.)

---

Anyway, you can enforce your constraints using xs:assert:

The use of attribute @xpathDefaultNamespace may not be necessary if your schema's namespace is null, but must be set to ##targetNamespace if you schema defines one, otherwise evaluation of the element name will fail.

Note: Obviously, for this to work it is necessary to use a validator which supports XML Schema 1.1. When using, say, OxygenXML, you can just set the default XML schema version to 1.1 in the XML / XML Parser / XML Schema preference page. When using Xerces as a validator, an additional feature must be activated: http://apache.org/xml/features/validation/cta-full-xpath-checking. Otherwise it will fail to evaluate most of the XPath you come up with because it uses an insufficient subset by default.

---