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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.

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>

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
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