How Can I Eliminate Redundant Namespace Declarations in Nested 'FOR XML PATH' Queries?

Nested Namespace Elimination in "FOR XML PATH" Queries

In the context of "FOR XML PATH" queries using "WITH XMLNAMESPACES" to define default namespaces, nested queries can introduce redundant namespace declarations at the top level. This issue arises when subqueries employ "FOR XML PATH" within the nested query.

Example:

Consider the following query:

...
select 
    a.c2 as "@species"
    , (select l.c3 as "text()" 
       from t2 l where l.c2 = a.c1 
       for xml path('leg'), type) as "legs"
from t1 a
for xml path('animal'), root('zoo')
...

Here, the nested query for "legs" will result in an additional namespace declaration for "uri:animal" on the "legs" element.

Solution:

To eliminate this redundancy, a combination of "FOR XML PATH" in subqueries and "FOR XML EXPLICIT" in the main query can be employed. This approach involves using the "xmltext" directive in the "FOR XML EXPLICIT" query to suppress the outermost namespace declaration.

DECLARE @xml XML = (SELECT OrderID AS "@OrderID",
                        (SELECT ItemID AS "@ItemID", 
                                Name AS "data()" 
                         FROM @OrderDetail 
                         WHERE OrderID = o.OrderID 
                         FOR XML PATH ('Item'), TYPE)
                    FROM @Order o 
                    FOR XML PATH ('Order'), ROOT('dummyTag'), TYPE)

SELECT 1 AS Tag
      ,NULL AS Parent
      ,@xml AS [xml!1!!xmltext]
      ,'http://test.com/order' AS [xml!1!xmlns]
FOR XML EXPLICIT

The result will be:

<xml xmlns="http://test.com/order">
  <Order OrderID="1">
    <Item ItemID="A">Drink</Item>
    <Item ItemID="B">Cup</Item>
  </Order>
  <Order OrderID="2">
    <Item ItemID="A">Drink</Item>
    <Item ItemID="C">Straw</Item>
    <Item ItemID="D">Napkin</Item>
  </Order>
</xml>

This technique provides the flexibility of "FOR XML PATH" for nested queries while maintaining a single namespace declaration on the root node.

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