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Guidelines and Limitations in Using XML Schema Collections on the Server

The XML schema definition language (XSD) validation has some limitations regarding SQL columns that use the xml data type. The following table provides details about those limitations and guidelines for modifying your XSD schema so it can work with SQL Server. The topics in this section provide additional information about specific limitations and guidance for working with them.

Item

Limitation

minOccurs and maxOccurs

The values for minOccurs and maxOccurs attributes must fit into 4-byte integers. Schemas that do not conform are rejected by the server.

<xsd:choice>

SQL Server rejects schemas that have an <xsd:choice> particle without children, unless the particle is defined with a minOccurs attribute value of zero.

<xsd:include>

Currently, SQL Server does not support this element. XML schemas that include this element are rejected by the server.

As a solution, XML schemas that include the <xsd:include> directive can be preprocessed to copy and merge the contents of any included schemas into a single schema for upload to the server. For more information, see Preprocessor Tool for XML Schemas.

<xsd:key>, <xsd:keyref>, and <xsd:unique>

Currently, SQL Server does not support these XSD-based constraints for enforcing uniqueness or establishing keys and key references. XML schemas that contain these elements cannot be registered.

<xsd:redefine>

SQL Server does not support this element. For information about another way to update schemas, see The <xsd:redefine> Element.

<xsd:simpleType> values

SQL Server only supports millisecond precision for simple types that have second components other than xs:time and xs:dateTime, and 100-nanosecond precision for xs:time and xs:dateTime. SQL Server puts limitations on all recognized XSD simple type enumerations.

SQL Server does not support using the "NaN" value in <xsd:simpleType> declarations.

For more information, seeValues for <xsd:simpleType> Declarations.

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.

xs:QName

SQL Server does not support types derived from xs:QName that use an XML Schema restriction element.

SQL Server does not support union types with xs:QName as a member element.

For more information, see The xs:QName Type.

Adding members to an existing substitution group

You cannot add members to an existing substitution group in an XML schema collection. A substitution group in an XML schema is restricted in that the head element and all its member elements must be defined in the same {CREATE | ALTER} XML SCHEMA COLLECTION statement.

Canonical forms and pattern restrictions

The canonical representation of a value cannot violate the pattern restriction for its type. For more information, see Canonical Forms and Pattern Restrictions.

Enumeration facets

SQL Server does not support XML schemas with types that have pattern facets or enumerations that violate those facets.

Facet length

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.

ID attribute

Each XML schema component can have an ID attribute on it. SQL Server enforces uniqueness for <xsd:attribute> declarations of ID type, but does not store these values. The scope for enforcement of uniqueness is the {CREATE | ALTER} XML SCHEMA COLLECTION statement.

ID type

SQL Server does not support elements of type xs:ID, xs:IDREF, or xs:IDREFS. A schema may not declare elements of this type, or elements derived by restriction or extension from this type.

Local namespace

The local namespace has to be explicitly specified for the <xsd:any> element. SQL Server rejects schemas that use an empty string ("") as a value for the namespace attribute. Instead, SQL Server requires the explicit use of "##local" to indicate an unqualified element or attribute as the instance of the wildcard character.

Mixed type and simple content

SQL Server does not support restricting a mixed type to a simple content. For more information, see Mixed Type and Simple Content.

NOTATION type

SQL Server does not support the NOTATION type.

Out-of-memory conditions

In working with large XML schema collections, an out-of-memory condition might occur. For solutions to this problem, see Large XML Schema Collections and Out-of-Memory Conditions.

Repeated values

SQL Server rejects schemas in which the block or final attribute has repeated values such as "restriction restriction" and "extension extension".

Schema component identifiers

SQL Server limits identifiers of schema components to a maximum length of 1000 Unicode characters. Also, surrogate character pairs within identifiers are not supported.

Time zone information

In SQL Server 2008, time zone information is fully supported for xs:date, xs:time, and xs:dateTime values for XML Schema validation. With SQL Server 2005 backwards-compatibility mode, time zone information is always normalized to Coordinated Universal Time (Greenwich Mean Time). For elements of dateTime type, the server converts the time provided to GMT by using the offset value ("-05:00") and returning the corresponding GMT time.

Union types

SQL Server does not support restrictions from union types.

Variable precision decimals

SQL Server does not support variable precision decimals. The xs:decimal type represents arbitrary precision decimal numbers. Minimally conforming XML processors must support decimal numbers with a minimum of totalDigits=18. SQL Server supports totalDigits=38, but limits the fractional digits to 10. All xs:decimal instanced values are represented internally by the server by using the SQL type numeric (38, 10).

In This Section

Topic

Description

Canonical Forms and Pattern Restrictions

Explains canonical forms and pattern restrictions.

Wildcard Components and Content Validation

Describes the limitations of using wildcard characters, lax validation, and anyType Elements with XML schema collections.

The <xsd:redefine> Element

Explains the limitation of using the <xsd:redefine> element and describes a workaround.

The xs:QName Type

Describes the limitation regarding the xs:QName type.

Values for <xsd:simpleType> Declarations

Describes the restrictions that are applied to <xsd:simpleType> declarations.

Enumeration Facets

Describes the limitation regarding enumeration facets.

Mixed Type and Simple Content

Describes the limitation on restricting a mixed type to a simple content.

Large XML Schema Collections and Out-of-Memory Conditions

Provides solutions for the out-of-memory condition that sometimes occurs with large schema collections.

Non-Deterministic Content Models

Describes the limitations regarding non-deterministic content models.