Question

Lets assume I have 2 XML variables in SQL Server 2008 with the following XML

DECLARE @FIRST XML = '<DBPerson>
                          <firstname>John</firstname>
                          <lastname>Bob</lastname>
                      </DBPerson>',
        @Second XML = '<FromUI>
                           <lastname>New Bob</lastname>
                           <age>39</age>
                       </FromUI>';

I want the following output:

<DBPerson>
     <firstname>John</firstname>
     <lastname>New Bob</lastname>
     <age>39</age>
</DBPerson>

Basically I want to merge contents of 2 XML variables into one where the variable @Second should hold precedence (if a node exists in both @First & @Second, the node inside @Second should be considered).

The approach I have taken is to first get the list of all unique elements within both the root elements like this:

WITH ALLFields AS
(
    SELECT 
          x.y.value('local-name(.)','varchar(50)') As Element   
    FROM @Second.nodes('FromUI/*') AS x(y)
    UNION
    SELECT 
          x.y.value('local-name(.)','varchar(50)') As Element   
    FROM @FIRST.nodes('DBPerson/*') AS x(y)
)
SELECT * FROM ALLFields AF

But I am clueless how to proceed from here on. I know I have to use sql:column somewhere to build a table first to get just node names and their values (based on AllFields) and then I can use FOR XML PATH('DBPerson') to form the final xml but a bit unsure of the usage of sql:column

Any help highly appreciated.

UPDATE:
I have boiled it down to the following query:

DECLARE @FIRST XML = '<DBPerson><firstname>John</firstname><lastname>Bob</lastname></DBPerson>',
        @Second XML = '<FromUI><lastname>New Bob</lastname><age>39</age></FromUI>';


WITH ALLFields AS
(
    SELECT 
          x.y.value('local-name(.)','varchar(50)') As Element   
    FROM @Second.nodes('FromUI/*') AS x(y)
    UNION
    SELECT 
          x.y.value('local-name(.)','varchar(50)') As Element   
    FROM @FIRST.nodes('DBPerson/*') AS x(y)
), Filtered AS
(
    SELECT 
          Element
        , @FIRST.value('(DBPerson/*[local-name()=sql:column("Element")])[1]','varchar(max)') AS F
        , @Second.value('(FromUI/*[local-name()=sql:column("Element")])[1]','varchar(max)') AS S
    FROM ALLFields AF
), FinalValues AS
(
    SELECT
        Element 
        , CASE 
                WHEN S IS NULL THEN F
                ELSE S
            END AS V
    FROM Filtered
)
SELECT * FROM FinalValues

This query gives me a table with all the elements in one column and the data for the elements in another column. Now how do i generate my final XML like this:

<DBPerson><firstname>John</firstname><lastname>New Bob</lastname><age>39</age></DBPerson>

Answer 1

select isnull(S.N.query('.'),F.N.query('.')) as '*'
from @First.nodes('/DBPerson/*') as F(N)
  full outer join @Second.nodes('/FromUI/*') as S(N)
    on F.N.value('local-name(.)', 'nvarchar(100)') = S.N.value('local-name(.)', 'nvarchar(100)')
for xml path(''), root('DBPerson')