Absense of targetNamespace causing exceptions to schema - xsd

Question

I'm trying to understand why the absence of a targetNamespace causes the following errors in my sample XML schema. If I remove the targetNamespace (targetNamespace="http://tempuri.org/XMLSchema1.xsd"), I get the following errors:
Namespace 'http://tempuri.org/XMLSchema1.xsd' is not
available to be referenced in this
schema. XMLSchema1.xsd 30
The 'http://tempuri.org/XMLSchema1.xsd:ChildNamePK' identity
constraint is not declared. XMLSchema1.xsd 30
If I add the targetNamespace back in, these errors go away but this is actually a sample from a sql annotated schema that I've shortened up for the purposes of this post. Since all the elements are local, I wanted to remove the targetNamespace in my real schema. How would I correct the schema without having to add a targetNamespace?
<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="XMLSchema1"
elementFormDefault="qualified"
xmlns="http://tempuri.org/XMLSchema1.xsd"
xmlns:mstns="http://tempuri.org/XMLSchema1.xsd"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Parent">
<xs:complexType>
<xs:sequence>
<xs:element name="Child" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="Name" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="OldestChild">
<xs:complexType>
<xs:sequence>
<xs:element name="Name" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:key name="ChildNamePK">
<xs:selector xpath=".//Child" />
<xs:field xpath="Name" />
</xs:key>
<xs:keyref name="OldestChildFK" refer="ChildNamePK">
<xs:selector xpath=".//OldestChild" />
<xs:field xpath="Name" />
</xs:keyref>
</xs:element>
</xs:schema>

Answer 1

The issue here is the line:
xmlns="http://tempuri.org/XMLSchema1.xsd"
This line says that any QNames that do not have a namespace prefix should be treated as if the namespace were http://tempuri.org/XMLSchema1.xsd. This is called the default namespace.
The refer attribute in the xs:keyref element is one of these QNames, and the value you have set it to is ChildNamePK which does not have a namespace prefix. Since there is no namespace prefix, and since you have defined a default namespace, XML schema effectively treats this as if it were
refer="{http://tempuri.org/XMLSchema1.xsd}ChildNamePK"
i.e. as if it referenced a ChildNamePK element in the http://tempuri.org/XMLSchema1.xsd namespace.
However, by removing the targetNamespace, you are saying the your elements, including the ChildNamePK key, are not in a namespace. So the thing that the refer attribute references doesn't actual exist. This is what the second error is trying to tell you--it's looking for http://tempuri.org/XMLSchema1.xsd:ChildNamePK' but it doesn't exist.
The right solution here is probably to just remove the xmlns="..." line so that there is no default namespace. That way the refer attribute will reference an a QName that is not in a namespace, which is exactly what ChildNamePK is when you remove the targetNamespace line.