Hello all,
my question follows, I am using XML Schema 1.0.

I have an attributeGroup with an optional attribute. This
attributeGroup is used in declaration of complexType. Another
complexType is then an extension of that complexType. Please see my
sample code below.

I would like to declare the attribute 'label' as required for the
'derived' complexType. Is there any legal way to do it?

<xsd:attributeGroup name="attributes">
    ...
    <xsd:attribute name="label" type="xsd:string" />
</xsd:attributeGroup>

<xsd:complexType name="parent">
    <xsd:complexContent>
        ...
        <xsd:attributeGroup ref="attributes"/>
    </xsd:complexContent>
</xsd:complexType>

<xsd:complexType name="derived">
    <xsd:complexContent>
        <xsd:extension base="parent">
            <xsd:sequence>
                <xsd:element ref="parameter" minOccurs="0"
maxOccurs="unbounded"/>
            </xsd:sequence>
            <xsd:attribute name="name" type="xsd:string" use="required"/>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

Regards,
Jan Pridal