How to do mutually exclusive attributes in XML schema?

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>

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...

Since RelaxNG was mentioned in Alnitak's answer, here is a solutionwith RelaxNG (a language which is, in most cases, better than W3CSchema). Do note the OR (|) in the definition of elem:

start = documentdocument = 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 messagesare suboptimal):

% rnv attributes-exclusive.rnc attributes-exclusive.xml    attributes-exclusive.xmlattributes-exclusive.xml:4:0: error: attribute ^ref not allowedrequired:       after% xmllint --noout --relaxng attributes-exclusive.rng attributes-exclusive.xmlattributes-exclusive.xml:4: element elem: Relax-NG validity error : Invalid attribute value for element elemattributes-exclusive.xml fails to validate