Limitations of the XML Schema collections

• SQL Server limits identifiers of schema components to a maximum length of 1000 Unicode characters. Also, surrogate character pairs within identifiers are not supported.
• SQL Server does not support restrictions from union types.
• The values for minOccurs and maxOccurs attributes must fit into 4-byte integers. Schemas that do not conform are rejected by the server.
• xsi:schemaLocation and xsi:noNamespaceSchemaLocation
  SQL Server ignores these attributes if they are present in the XML instance data inserted into a column or variable of xml data type.
• SQL Server rejects schemas that have an particle without children, unless the particle is defined with a minOccurs attribute value of zero.
• Currently, SQL Server does not support . XML schemas that include this element are rejected by the server.
• Currently, SQL Server does not support <xsd:key>, <xsd:keyref>, and <xsd:unique> constraints for enforcing uniqueness or establishing keys and key references. XML schemas that contain these elements cannot be registered.
• The length, minLength, and maxLength facets are stored as a long type. This type is a 32-bit type. Therefore, the range of acceptable values for these values is 2^31.
• SQL Server does not support the NOTATION type.
• SQL Server does not support types derived from xs:QName that use an XML Schema restriction element and the union types with xs:QName as a member element.
• Members cannot be added to an existing substitution group in an XML schema collection.
• The canonical representation of a value cannot violate the pattern restriction for its type.
• SQL Server does not support XML schemas with types that have pattern facets or enumerations that violate those facets.
• In working with large XML schema collections, an out-of-memory condition might occur.
• SQL Server rejects schemas in which the block or final attribute has repeated values such as "restriction restriction" and "extension extension". . 

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