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Compare Typed XML to Untyped XML

You can create variables, parameters, and columns of the xml type. You can optionally associate a collection of XML schemas with a variable, parameter, or column of xml type. In this case, the xml data type instance is called typed. Otherwise, the XML instance is called untyped.

The xml data type implements the ISO standard xml data type. Therefore, it can store well-formed XML version 1.0 documents and also so-called XML content fragments with text nodes and an arbitrary number of top-level elements in an untyped XML column. The system checks that the data is well-formed, does not require the column to be bound to XML schemas, and rejects data that is not well-formed in the extended sense. This is true also of untyped XML variables and parameters.

An XML schema provides the following:

  • Validation constraints. Whenever a typed xml instance is assigned to or modified, SQL Server validates the instance.

  • Data type information. Schemas provide information about the types of attributes and elements in the xml data type instance. The type information provides more precise operational semantics to the values contained in the instance than is possible with untyped xml. For example, decimal arithmetic operations can be performed on a decimal value, but not on a string value. Because of this, typed XML storage can be made significantly more compact than untyped XML.

Use untyped xml data type in the following situations:

  • You do not have a schema for your XML data.

  • You have schemas, but you do not want the server to validate the data. This is sometimes the case when an application performs client-side validation before storing the data at the server, or temporarily stores XML data that is invalid according to the schema, or uses schema components that are not supported at the server.

Use typed xml data type in the following situations:

  • You have schemas for your XML data and you want the server to validate your XML data according to the XML schemas.

  • You want to take advantage of storage and query optimizations based on type information.

  • You want to take better advantage of type information during compilation of your queries.

Typed XML columns, parameters, and variables can store XML documents or content. However, you have to specify with a flag whether you are storing a document or content at the time of declaration. Additionally, you have to provide the collection of XML schemas. Specify DOCUMENT if each XML instance has exactly one top-level element. Otherwise, use CONTENT. The query compiler uses the DOCUMENT flag in type checks during query compilation to infer singleton top-level elements.

Before you can create typed xml variables, parameters, or columns, you must first register the XML schema collection by using CREATE XML SCHEMA COLLECTION (Transact-SQL). You can then associate the XML schema collection with variables, parameters, or columns of the xml data type.

In the following examples, a two-part naming convention is used for specifying the XML schema collection name. The first part is the schema name, and the second part is the XML schema collection name.

Example: Associating a Schema Collection with an xml Type Variable

The following example creates anxml type variable and associates a schema collection with it. The schema collection specified in the example is already imported in the AdventureWorks database.

DECLARE @x xml (Production.ProductDescriptionSchemaCollection); 

Example: Specifying a Schema for an xml Type Column

The following example creates a table with an xml type column and specifies a schema for the column:

 Col1 int, 
 Col2 xml (Production.ProductDescriptionSchemaCollection)) ;

Example: Passing an xml Type Parameter to a Stored Procedure

The following example passes an xml type parameter to a stored procedure and specifies a schema for the variable:

  @ProdDescription xml (Production.ProductDescriptionSchemaCollection) 

Note the following about the XML schema collection:

  • An XML schema collection is available only in the database in which it was registered by using Creating an XML Schema Collection.

  • If you cast from a string to a typed xml data type, the parsing also performs validation and typing, based on the XML schema namespaces in the collection specified.

  • You can cast from a typed xml data type to an untyped xml data type, and vice versa.

For more information about other ways to generate XML in SQL Server, see Create Instances of XML Data. After XML is generated, it can be assigned either to an xml data type variable or stored in xml type columns for additional processing.

In the data type hierarchy, the xml data type appears below sql_variant and user-defined types, but above any of the built-in types.

Example: Specifying Facets to Constrain a Typed xml Column

For typed xml columns, you can constrain the column to allow only single, top-level elements for each instance stored in it. You do this by specifying the optional DOCUMENT facet when a table is created, as shown in the following example:

   (DOCUMENT Production.ProductDescriptionSchemaCollection));

By default, instances stored in the typed xml column are stored as XML content and not as XML documents. This allows for the following:

  • Zero or many top-level elements

  • Text nodes in top-level elements

You can also explicitly specify this behavior by adding CONTENT facet, as shown in the following example:

CREATE TABLE T(Col1 xml(CONTENT Production.ProductDescriptionSchemaCollection));
GO -- Default

Note that you can specify the optional DOCUMENT/CONTENT facets anywhere you define xml type (typed xml). For example, when you create a typed xml variable, you can add the DOCUMENT/CONTENT facet, as shown in the following:

declare @x xml (DOCUMENT Production.ProductDescriptionSchemaCollection);

The xml data type columns, variables, and parameters can be typed by using XML schema, but not by using DTD. However, inline DTD can be used for both untyped and typed XML to supply default values and to replace entity references with their expanded form.

You can convert DTDs to XML schema documents by using third-party tools, and load the XML schemas into the database.

SQL Server 2008 made several extensions to the XML Schema support, including support for lax validation, improved handling of xs:date, xs:time and xs:dateTime instance data, and added support for list and union types. In most cases the changes do not affect the upgrade experience. However if you used an XML Schema collection in SQL Server 2005 that allowed values of type xs:date, xs:time, or xs:dateTime (or any subtype) then the following upgrade steps occur when you attach your SQL Server 2005 database to a later version of SQL Server:

  1. For every XML column, that is typed with an XML Schema Collection that contains elements or attributes that are typed as either xs:anyType, xs:anySimpleType, xs:date or any of its subtypes, xs:time or any subtype thereof, or xs:dateTime or any of its subtypes, or are union or list types containing any of these types the following occurs:

    1. All XML indices on the column will be disabled.

    2. All SQL Server 2005 values will continue to be represented in the Z timezone, because they have been normalized to the Z timezone.

    3. Any xs:date or xs:dateTime values that are smaller than January 1st of year 1 will lead to a runtime error when the index gets rebuild or an XQuery or XML-DML statements gets executed against the XML data type containing that value.

  2. Any negative years in xs:date or xs:dateTime facets or default values in an XML Schema collection will automatically be updated to the smallest value allowed by the base xs:date or xs:dateTime type (e.g., 0001-01-01T00:00:00.0000000Z for xs:dateTime).

Note that you can still use a simple SQL select statement to retrieve the whole XML data type, even if it contains negative years. It is recommended that you replace negative years with a year within the newly supported range or change the type of the element or attribute to xs:string.

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