> This is my first time to have a discussion here. I'm new to 
> XML Schema.
> Now I want to define an element with three attributes: A, B 
> and C. However, I don't want the three attributes occur 
> simultaneously.
> I need A and B occur simultaneously, or only C occurs singlely.

I'm afraid it's a well-known limitation of XSD 1.0 that you can't do this.
It always surprises newcomers, but I'm afraid it's true. The limitation
disappears in XSD 1.1, but that's still "on the drawing board" and not yet
supported in most products.

If you want to try it out, Saxon-SA 9.1 does support the new feature of
"conditional type assignment". However, the "mainstream" solution to this
kind of requirement is to supplement XSD validation with another validation
pass using Schematron, which allows testing your document against arbitrary
predicates.

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay 


> 
> <x A="a" B="b"/>
> or
> <x C="c">
>  	 EG320270420CN
> How to do this?
> I use 'choic' to implement this as follows, but it don't work.
> <xs:choice maxOccurs="unbounded">
>       <xs:element name="x"  type="xs:string"> 
> <xs:complexType> <xs:attribute name="A" type="xs:string"/> 
> <xs:attribute name="B" type="xs:string"/> </xs:complexType>
>       </xs:element>
>       <xs:element name="x"  type="xs:string"> 
> <xs:complexType> <xs:attribute name="C" type="xs:string"/> 
> </xs:complexType>
>       </xs:element>
> </xs:choice>
> By the way, after goooooogling, I found that the code above 
> is right in the old XML Schema criteria, but nowadays I can't 
> use it any more and the XMLSpy software will report errors 
> with the code above. Is there anyone that know about the 
> related information?
> It has bothered me several days. I'll appreciate your help sincerely.
> 
> 
> 
>