The constraint you are looking for can not be achieved using XSD schemas. 
Only work around is using xsi:type attribute[1] to define the type of the 
element in the instance document and defining different types in the schema 
as you need it. 

Although Schematron supports these kind of constraints, Microsoft xml 
validators do not support Schematron at this time.

[1] http://www.w3.org/TR/xmlschema-1/#xsi_type

-- 
S. Huseyin Ulger [MSFT]
MSXML Dev


"Duarte S" wrote:

> Hi,
> Is it possible, in a W3C schema, to constraint the structure of
> elements based on attribute values? For example:
> 
> <instr type="sql" instruction="SELECT * ....." />
> <instr type="exec" command="C:/work.bat" parameters="/p /s" />
> 
> In this example, the attribute 'type' is allways present, however the
> other attributes depend on the value of the 'type' attribute
> ('instruction' is only present if type="sql", etc...). Is it possible
> to apply such a constraint within a W3C Schema?
> 
> I've read in another topic that this kind of constraint could be made
> with a Schematron Schema. If so, is it possible to do a W3C Schema with
> embedded Schematron rules? How are these rules applied?
> 
> Thanks
> 
> Duarte
> 
>