How to do mutually exclusive attributes in XML schema?

Question

I'm trying to make two XML attributes to be mutually exclusive. How can one create an XSD schema to capture this kind of scenario?

I would like to have one of these

<elem value="1" />
<elem ref="something else" />

but not

<elem value="1" ref="something else" />

Answer 1

Unfortunately AFAIK you can't do that with XML Schema, I've had the same problem myself.

I've seen it suggested that if you need both of:

<elem type="xxx"> 
<elem ref="yyy">

then <elem> itself should be split into two types, since they've clearly got different attributes...

Answer 2

You can't do with attributes, but you can with child elements...

<element name="elem">
    <complexType>
     <choice>
      <element name="value"/>
      <element name="ref"/>
     </choice>
    </complexType>
</element>

This way you can have...

<elem>
    <value>1</value>
</elem>

or...

<elem>
    <rel>something else</rel>
</elem>

Answer 3

Since RelaxNG was mentioned in Alnitak's answer, here is a solution with RelaxNG (a language which is, in most cases, better than W3C Schema). Do note the OR (|) in the definition of elem:

start = document
document = element document {elem+}
elem = element elem {ref | value}
ref = attribute ref {text}
value = attribute value {xsd:integer}

If I have this XML file:

<document>
    <elem value="1" />
    <elem ref="something else" />
</document>

It is accepted by rnv and xmlint:

 % rnv attributes-exclusive.rnc attributes-exclusive.xml             
 attributes-exclusive.xml

 % xmllint --noout --relaxng attributes-exclusive.rng attributes-exclusive.xml 
 attributes-exclusive.xml validates

If I add in the XML file:

<elem value="1" ref="something else" />

I get validation errors, as I want (do note that the error messages are suboptimal):

% rnv attributes-exclusive.rnc attributes-exclusive.xml    
attributes-exclusive.xml
attributes-exclusive.xml:4:0: error: attribute ^ref not allowed
required:
       after

% xmllint --noout --relaxng attributes-exclusive.rng attributes-exclusive.xml
attributes-exclusive.xml:4: element elem: Relax-NG validity error : Invalid attribute value for element elem
attributes-exclusive.xml fails to validate