Ensure child element cannot exist without parent Using Xsd - xsd

Question

I am facing issues in trying to get this schema validation inside xsd.
Valid case
<root>
<groups>
<group/>
</groups>
</root>
Valid case
<root>
<groups/>
</root>
Invalid case
<root>
<group/>
</root>
How to ensure that a particular child element can only exist under some parent, not alone in xml through xsd?
In this e.g.
group cannot exist alone, but can exist when groups is parent...
Someone replied that do not make group element global i.e contain it within groups element...
But there can be a case,
where group is not the direct child of parent.
e.g
valid case
<groups>
<class>
<group>
</class>
</groups>
what should be done in that cases...as class needs to refer to group too...

Answer 1

Based on your question, it sounds to me more like you want class and group to be interchangeable. For that, you want to use a recursive schema element, like so...
<xsd:element name="Groups">
<xsd:complexType>
<xsd:complexContent>
<xsd:extension base="BranchType"/>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
<xsd:complexType name="BranchType">
<xsd:sequence>
<xsd:choice minOccurs="0" maxOccurs="unbounded">
<xsd:element name="Class" type="BranchType" minOccurs="0" maxOccurs="1"/>
<xsd:element name="Group" minOccurs="0" maxOccurs="1"/>
</xsd:choice>
</xsd:sequence>
</xsd:complexType>
We basically define a BranchType that can contain any mix of Group elements or itself (via the Class element). Then we define the top level Groups as type BranchType. I use a sequence of choice so that the Class and Group elements can appear in any order, any number of times, to any level of nesting.

Answer 2

When you don't want an element to show by itself, it must be declared inside the complex type that defines its parent.
Below is an easy "fix", generated by a tool; when learning, it may prove a good way to jumpstart XSDs, at least to get the syntax right...
UPDATE: If you keep adding content, you need to update the schema. If the element is not global, then the solution is the same: definition within its parent. If the content model is the same, then define that as a global type, and refer it in the definition of the element (in this case it would be instead of anyType).
<?xml version="1.0" encoding="utf-8"?>
<!--XML Schema generated by QTAssistant/XML Schema Refactoring (XSR) Module (http://www.paschidev.com)-->
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="root">
<xs:complexType>
<xs:sequence>
<xs:element name="groups">
<xs:complexType>
<xs:sequence minOccurs="0">
<xs:element name="group" type="xs:anyType" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="groups">
<xs:complexType>
<xs:sequence>
<xs:element name="class">
<xs:complexType>
<xs:sequence>
<xs:element name="group" type="xs:anyType" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>