Element reference in 2 elements - xsd

Question

I don't know how to title my question.
I have an XSD which contains the following elements
<xs:element name="abc">
<xs:complexType>
<xs:element maxOccurs="unbounded" ref="ele1"/>
</xs:complexType>
</xs:element>
<xs:element name="xyz">
<xs:complexType>
<xs:element maxOccurs="unbounded" ref="ele1"/>
</xs:complexType>
</xs:element>
<xs:element name="ele1">
<xs:complexType>
<xs:attribute name="ID" type="xs:integer"/>
</xs:complexType>
</xs:element>
The question is for element xyz ID is mandatory whereas for abc it is not; how can I specify this in the XSD?

Answer 1

Assuming ID is a unique identifier for the content repeated under the "container", then you could set a xs:key constraint for the xyz like this:
<?xml version="1.0" encoding="utf-8" ?>
<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="abc">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" ref="ele1"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="xyz">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" ref="ele1"/>
</xs:sequence>
</xs:complexType>
<xs:key name="PK_xyz">
<xs:selector xpath="ele1"/>
<xs:field xpath="#ID"/>
</xs:key>
</xs:element>
<xs:element name="ele1">
<xs:complexType>
<xs:attribute name="ID" type="xs:integer"/>
</xs:complexType>
</xs:element>
</xs:schema>
Then, invalid XML because ID is missing:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!-- Sample XML generated by QTAssistant (http://www.paschidev.com) -->
<xyz xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ele1 ID="1"/>
<ele1 />
</xyz>
Error message:
Error occurred while loading [], line 5 position 3
The identity constraint 'PK_xyz' validation has failed. Either a key is missing or the existing key has an empty node.
Document3.xml is invalid.
Invalid because it is duplicate (this is the downside, if the above assumption on uniqueness doesn't stand true):
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!-- Sample XML generated by QTAssistant (http://www.paschidev.com) -->
<xyz xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ele1 ID="1"/>
<ele1 ID="1"/>
</xyz>
Error:
Error occurred while loading [], line 5 position 3
There is a duplicate key sequence '1' for the 'PK_xyz' key or unique identity constraint.
Document3.xml is invalid.
Working sample:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!-- Sample XML generated by QTAssistant (http://www.paschidev.com) -->
<xyz xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ele1 ID="1"/>
<ele1 ID="2"/>
</xyz>