XSD modelling question

I need to define the Key constraint that consist of
{@id, @name, sg/name} for the below xml instance-

<data>
  <sgs>
     <owner id="a" name="b">  <!-- part of sgKey -->
 	<sg>
 	  <name>sg1</name> <!-- part of sgKey -->
 	  <description/>
 	</sg>
        <sg> <name>sg2</name> <description/> </sg>
        <sg> <name>sg6</name> <description/> </sg>
        <!-- unbounded list of 'sg' elements -->
     </owner>
     <!-- unbounded list of 'owner' elements -->
  </sgs>
</data>

Let's say, I define this sgKey at 'sgs' element scope
as- 
   Selector= owner
   field=@id
   field=@name
   field=sg/name

But this would be an invalid Key definition because
there will be Multiple 'sg/name' children for a given
'owner'. How would I define the Key in this situation.

Thanks for the tip.
    
-D


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