Different elements depending on the value of an attribute

We have developed an XML application and now I'm trying to write an schema 
to validate the incoming requests. The problem is that I don't know if it 
is possible to write an schema reflecting the way we are working: we have a 
set of possible requests and then we send a response. The type of each 
request is set with the attribute "type", and depending of this type, the 
content of the request must be one or another. For example:
<request type="user data">
     <user_id>test</user_id>
     <password>1234</password>
</request>
<request type="users list">
     <country>SPAIN</country>
</request>

I can't find a way to define a suitable schema. Could someone help? thanks.