Change Existing Columns to XML Columns

The ALTER TABLE statement supports the xml data type. For example, you can alter any string type column to the xml data type. Note that in these cases, the documents contained in the column must be well formed. Also, if you are changing the type of the column from string to typed xml, the documents in the column are validated against the specified XSD schemas.

CREATE TABLE T (Col1 int primary key, Col2 nvarchar(max))
GO
INSERT INTO T 
VALUES (1, '<Root><Product ProductID="1"/></Root>')
GO
ALTER TABLE T 
ALTER COLUMN Col2 xml
GO

You can change an xml type column from untyped XML to typed XML. For example:

CREATE TABLE T (Col1 int primary key, Col2 xml)
GO
INSERT INTO T 
values (1, '<p1:ProductDescription ProductModelID="1" 
xmlns:p1="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription">
            </p1:ProductDescription>')
GO 
-- Make it a typed xml column by specifying a schema collection.
ALTER TABLE T 
ALTER COLUMN Col2 xml (Production.ProductDescriptionSchemaCollection)
GO

In the previous example, all the instances stored in the column are validated and typed against the XSD schemas in the specified collection. If the column contains one or more XML instances that are invalid with regard to the specified schema, the ALTER TABLE statement will fail and you will not be able to change your untyped XML column into typed XML.

For more information about typed XML, see Compare Typed XML to Untyped XML.