XSD unique constraint on attribute of sibling elements of a specific type

Question

I have an XML document structured as Q&A which follows the following format (edited for clarity):

    
        &l         


        

Answer 1

Just change the selector to and you'll be fine. In general it is good to avoid XPaths like .//*, if only for performance reasons.

This is the XML Schema for the XML sample you've provided that I think is working the way you want:

Your posted XML validates fine with the above; duplicating any sibling answer's id yields a validation error. For example, the following XML:

When validated (in QTAssistant, should be similar to the message in Visual Studio since it is based on the same technology), these are the errors:

Error occurred while loading [], line 6 position 5
There is a duplicate key sequence '1' for the 'AnswerIdUnique' key or unique identity constraint.
Error occurred while loading [], line 9 position 3
There is a duplicate key sequence '1' for the 'AnswerIdUnique' key or unique identity constraint.
Document1.xml is invalid.

Below is a screenshot from Visual Studio 2010 showing the above XML validation against the XSD I've posted; while the problems are inadvertently reported as warnings, they are, nonetheless, reported.

I've randomly picked an online validator (http://xsdvalidation.utilities-online.info/) and validated the same XML and XSD I've posted; the error is reported as:

org.xml.sax.SAXParseException: Duplicate unique value [1] declared for identity constraint of element "question".org.xml.sax.SAXParseException: Duplicate unique value [1] declared for identity constraint of element "question".

One thing you have to pay attention to is when you have a target namespace for your XSD; in that case, it is needed to define an alias for all of the involved namespaces, and use them in your selectors.

UPDATE: And the XSD with namespaces:

Please notice the introduction of the tns prefix and the use of it in the selector.