Non-Deterministic Content Models

Before SQL Server 2005 Service Pack 1 (SP1), SQL Server rejected XML schemas that had non-deterministic content models.

Beginning with SQL Server 2005 SP1, however, non-deterministic content models are accepted if the occurrence constraints are 0,1, or unbounded.

Example: Non-deterministic content model rejected

The following example attempts to create an XML schema with a non-deterministic content model. The code fails because it is not clear whether the <root> element should have a sequence of two <a> elements or if the <root> element should have two sequences, each with an <a> element.

CREATE XML SCHEMA COLLECTION MyCollection AS '  
<schema xmlns="http://www.w3.org/2001/XMLSchema">  
    <element name="root">  
        <complexType>  
            <sequence minOccurs="1" maxOccurs="2">  
                <element name="a" type="string" minOccurs="1" maxOccurs="2"/>  
            </sequence>  
        </complexType>  
    </element>  
</schema>  
'  
GO  

The schema can be fixed by moving the occurrence constraint to a unique location. For example, the constraint can be moved to the containing sequence particle:

<sequence minOccurs="1" maxOccurs="4">  
    <element name="a" type="string" minOccurs="1" maxOccurs="1"/>  
</sequence>  

Or the constraint can be moved to the contained element:

<sequence minOccurs="1" maxOccurs="1">  
     <element name="a" type="string" minOccurs="1" maxOccurs="4"/>  
</sequence>  

Example: Non-deterministic content model accepted

The following schema would be rejected in versions of SQL Server before SQL Server 2005 SP1.

CREATE XML SCHEMA COLLECTION MyCollection AS '  
<schema xmlns="http://www.w3.org/2001/XMLSchema">  
    <element name="root">  
        <complexType>  
            <sequence minOccurs="0" maxOccurs="unbounded">  
                <element name="a" type="string" minOccurs="0" maxOccurs="1"/>  
                <element name="b" type="string" minOccurs="1" maxOccurs="unbounded"/>  
            </sequence>  
        </complexType>  
    </element>  
</schema>  
'  
GO  

See Also

Requirements and Limitations for XML Schema Collections on the Server