Question

How can I update the value of an xml tag with the value of an xml tag from another related table?

something like this:

UPDATE v2
 SET
 [xml].modify ('replace value of (//TAG1/text())[1] 
                with "CAST(v1.[xml].query(''//TAG2'') AS NVARCHAR(MAX))"')
FROM 
 table2 v2, 
 table1 v1 
WHERE
 v2.id = v1.id

Answer 1

I don’t think you can do this in a single step – but you can do it in two steps, if you’re on SQL Server 2008:

DECLARE @NewValue NVARCHAR(50)

SELECT @NewValue = [xml].value('(//TAG2)[1]', 'NVARCHAR(50)')
FROM dbo.v1 
WHERE id = 1

UPDATE dbo.v2
SET [xml].modify('replace value of (//TAG1/text())[1] with sql:variable("@NewValue")')
WHERE id = 1

The ability to specify a sql:variable in your replace value of XQuery is a new feature in SQL Server 2008 – so if you’re stuck on 2005, this won’t work, unfortunately.