Hi,

Here it is a schema that implements 1 and 2:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified">
  <xs:element name="language">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="combinations"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="combinations">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" ref="combination"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="combination">
    <xs:complexType>
      <xs:sequence>
        <xs:element minOccurs="2" maxOccurs="2" ref="element"/>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="uniqueElements">
      <xs:selector xpath="element"></xs:selector>
      <xs:field xpath="."></xs:field>
    </xs:unique>
  </xs:element>
  <xs:element name="element" type="xs:NMTOKEN"/>
</xs:schema>

Point 1 is implemented with maxOccurs="unbounded" in
<xs:element maxOccurs="unbounded" ref="combination"/>
and point 2 is implemented with a unique constraint on the combination
element:
    <xs:unique name="uniqueElements">
      <xs:selector xpath="element"></xs:selector>
      <xs:field xpath="."></xs:field>
    </xs:unique>

Point 3 cannot be implemented with a unique constraint because the
XPath used in XML Schema does not allow predicates and one cannot refer
to the first element and the second element in a compound constraint,
that is in a constraint with two fields.
Possible approaches further are to embed Schematron rules to check that
constraint
...
 <xs:element name="combination">
    <xs:annotation>
      <xs:appinfo>
        <pattern xmlns="http://www.ascc.net/xml/schematron"
name="uniquePairTest">
          <rule context="combination">
            <assert
test="not(preceding::combination[element[1]=current()/element[1] and
element[2]=current()/element[2]])">
              We should not have duplicate combinations.
            </assert>
          </rule>
        </pattern>
      </xs:appinfo>
    </xs:annotation>
    <xs:complexType>
...

or to use different names for the first element and for the second
element, for instance element1 and element2:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified">
  <xs:element name="language">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="combinations"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="combinations">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" ref="combination"/>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="uniqueCombination">
      <xs:selector xpath="combination"></xs:selector>
      <xs:field xpath="element1"/>
      <xs:field xpath="element2"/>
    </xs:unique>
  </xs:element>
  <xs:element name="combination">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="element1"/>
        <xs:element ref="element2"/>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="uniqueElements">
      <xs:selector xpath="element1|element2"></xs:selector>
      <xs:field xpath="."></xs:field>
    </xs:unique>
  </xs:element>
  <xs:element name="element1" type="xs:NMTOKEN"/>
  <xs:element name="element2" type="xs:NMTOKEN"/>
</xs:schema>

Best Regards,
George
---------------------------------------------------------------------
George Cristian Bina
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
www.---.com


Nichino wrote:
> Hello,
> I'm trying to define my first Xml Schema and have a problem.
> The XML I'd like to define through the Schema is the following one.
> Can you help me please? I don't know how to insert the 3 final notes.
> Many thanks and my apologies for the trouble.
>
> Best,
> Nick
>
> <language>
> <combinations>
> <combination>
> <element> .... </element>
> <element> .... </element>
> </combination>
> <combination>
> <element> .... </element>
> <element> .... </element>
> </combination>
> <combination>
> <element> .... </element>
> <element> .... </element>
> </combination>
> </combinations>
>
> </language>
>
> Note
> 1. There is no limit to the number of combinations.
> 2. Two elements having the same content can't be combined.
>
> E.g.
>
> <combination>
> <element> pippo </element>
> <element> pippo </element>
> </combination>
>
> it's not valid.
>
> 3. it's not possible to insert two combinations having the same elements.
>
> E.g.
>
> <combinations>
> <combination>
> <element> pippo </element>
> <element> pluto </element>
> </combination>
> <combination>
> <element> pluto </element>
> <element> pippo </element>
> </combination>
> </combinations>
> 
> it's not valid.