Restrict global element to be used outside/instead of the root element - xsd

Question

I have defined some global elements in my xsd schema. Now if the xml file contains any of the global elements alone(without any other root element) the validation should not allow this. But in my case if only the global element is present, the file is validated successfully.
In my case there is GROUP element which is a global element. This element should always be contained in a CASE element but if remove the CASE element the xml file is still validated successfully.
I cannot make the GROUP element as a local element as it has some complex logic(recursive calls to itself) and used at more than one place. Moreover when I tried to make it as local it gave me errors.
Now is there any solution to restrict it not to be used outside the CASE element.
Example(Allowed):
<?xml version="1.0" encoding="utf-8" ?>
<CASE>
<GROUP>
</GROUP>
</CASE>
Not Allowed:
<?xml version="1.0" encoding="utf-8" ?>
<GROUP>
</Group>

Answer 1

I'm sorry but there is no way in XML Schema to specify which global element is the only one allowed as a root. You may need to look at other techniques to validate this requirement (such as embedding Schematron rules

Answer 2

You could try a workaround: wrap your <GROUP> element inside a <xs:group name="someName"> element so that this group contains only your <GROUP> element. (eh, a bit confusing because of these names...) In this case, whenever you need to refer to your global <GROUP> element, you should refer to that "someName" group instead.
This workaround may not be applicable for you if your recursive element definition is complicated.

Answer 3

Have you tried to make the Group element local, but giving it a global type to allow recursion and reuse?
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Case" type="CaseType"/>
<xs:complexType name="CaseType">
<xs:sequence>
<xs:element name="Group" type="GroupType" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="GroupType">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="Group" type="GroupType"/>
<xs:element name="Case" type="CaseType"/>
</xs:choice>
</xs:complexType>
</xs:schema>