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requirements-and-limitations-for-xml-schema-collections-on-the-server.md

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title description author ms.author ms.reviewer ms.date ms.service ms.subservice ms.topic helpviewer_keywords
Requirements and limitations (XML schema collections)
Learn about the requirements and limitations for modifying your XML schema collections in SQL Server.
MikeRayMSFT
mikeray
randolphwest
05/05/2022
sql
xml
conceptual
identifiers [XML schema collections]
XML schema collections [SQL Server], limitations
substitution groups [XML in SQL Server]
XML schema collections [SQL Server], guidelines
lax validation
enumeration facets [XML in SQL Server]
decimal precision [XML in SQL Server]
repeated XML schema collection values
schema collections [SQL Server], limitations
time zones [XML in SQL Server]
precision decimals [XML in SQL Server]
schema collections [SQL Server], guidelines
lexical representation

Requirements and limitations for XML schema collections on the server

[!INCLUDE SQL Server Azure SQL Database]

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 [!INCLUDEssNoVersion]. The articles 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 don't conform are rejected by the server.
<xsd:choice> [!INCLUDEssNoVersion] 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, [!INCLUDEssNoVersion] doesn't 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 Preprocess a Schema to Merge Included Schemas.
<xsd:key>, <xsd:keyref>, and <xsd:unique> Currently, [!INCLUDEssNoVersion] doesn't support these XSD-based constraints for enforcing uniqueness or establishing keys and key references. XML schemas that contain these elements can't be registered.
<xsd:redefine> [!INCLUDEssNoVersion] doesn't support this element. For information about another way to update schemas, see The <xsd:redefine> Element.
<xsd:simpleType> values [!INCLUDEssNoVersion] 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. [!INCLUDEssNoVersion] puts limitations on all recognized XSD simple type enumerations.

[!INCLUDEssNoVersion] doesn't support using the "NaN" value in <xsd:simpleType> declarations.

For more information, see Values for <xsd:simpleType> Declarations.
xsi:schemaLocation and xsi:noNamespaceSchemaLocation [!INCLUDEssNoVersion] ignores these attributes if they are present in the XML instance data inserted into a column or variable of xml data type.
xs:QName [!INCLUDEssNoVersion] doesn't support types derived from xs:QName that use an XML Schema restriction element.

[!INCLUDEssNoVersion] doesn't 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 can't 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 can't violate the pattern restriction for its type. For more information, see Canonical Forms and Pattern Restrictions.
Enumeration facets [!INCLUDEssNoVersion] doesn't 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. [!INCLUDEssNoVersion] enforces uniqueness for <xsd:attribute> declarations of ID type, but doesn't store these values. The scope for enforcement of uniqueness is the {CREATE | ALTER} XML SCHEMA COLLECTION statement.
ID type [!INCLUDEssNoVersion] doesn't 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. [!INCLUDEssNoVersion] rejects schemas that use an empty string ("") as a value for the namespace attribute. Instead, [!INCLUDEssNoVersion] 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 [!INCLUDEssNoVersion] doesn't support restricting a mixed type to a simple content. For more information, see Mixed Type and Simple Content.
NOTATION type [!INCLUDEssNoVersion] doesn't support the NOTATION type.
Out-of-memory conditions When 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 [!INCLUDEssNoVersion] rejects schemas in which the block or final attribute has repeated values such as "restriction restriction" and "extension extension".
Schema component identifiers [!INCLUDEssNoVersion] 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 [!INCLUDEsql2008-md] and later versions, time zone information is fully supported for xs:date, xs:time, and xs:dateTime values for XML Schema validation. With [!INCLUDEssVersion2005] 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 [!INCLUDEssNoVersion] doesn't support restrictions from union types.
Variable precision decimals [!INCLUDEssNoVersion] doesn't 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. [!INCLUDEssNoVersion] 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).

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