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Although the XML datatype is treated like many other datatypes in SQL Server 2005, there are specific limitations to how it is used. These limitations are:

• XML types cannot convert to text or ntext data types.
• No data type other than one of the string types can be cast to XML.
• XML columns cannot be used in GROUP BY statements.
• Distributed partitioned views or materialized views cannot contain XML data types.
• Use of the sql_variant instances cannot include XML as a subtype.
• XML columns cannot be part of a primary or foreign key.
• XML columns cannot be designated as unique.
• Collation (COLLATE clause) cannot be used on XML columns.
• XML columns cannot participate in rules.
• The only built-in scalar functions that apply to XML columns are ISNULL and COALESCE. No other scalar built-in functions are supported for use against XML types.
• Tables can have only 32 XML columns.
• Tables with XML columns cannot have a primary key with more than 15 columns.
• Tables with XML columns cannot have a timestamp data type as part of their primary key.
• Only 128 levels of hierarchy are supported within XML stored in the database.