XML Schema how to make sub-elements unique for each element?

What i want to accomplish with my xml schema is to make questions unique per questionPool - not for all questionPoo开发者_如何学JAVAls. In my xml file i have this code, comments say what i want to do:

<questionPools>
    <questionPool>
        <question idref="question1"/>
        <question idref="question2"/>
    </questionPool>
    <questionPool>
        <question idref="question3"/>
        <question idref="question1"/>    <!-- Duplicate but not in question pool so allowed -->
        <question idref="question3"/>    <!-- Not allowed because duplicate in question pool -->
    </questionPool>
</questionPools>

I have tried this but it checks all questions rather then ones specific to one pool:

<xsd:key name="keyQuestionPool">
    <xsd:selector xpath="questionPools/questionPool/question"/>
    <xsd:field    xpath="@idref"/>
</xsd:key>

The scope of the key is the containing element. Put the <xs:key> definition as the child of the <xs:element name="questionPool"> element definition and drop the first steps from the XPath of the selector to match the new context.