How to Eliminate Redundant Namespace Declarations in Nested FOR XML PATH Queries?

Eliminating Redundant Namespace Declarations in Nested FOR XML PATH Queries

In FOR XML PATH queries with default namespaces declared using WITH XMLNAMESPACES, it's common to encounter duplicated namespace declarations in nested queries. This article addresses this issue and provides an optimal solution.

The problem arises when subqueries use FOR XML and inherit the default namespace from the outer query. This results in multiple namespace declarations for the subquery's nodes. While there are various online solutions, none fully resolve this problem.

The Optimal Solution

After extensive experimentation, the following solution has been deemed the most effective:

DECLARE @Order TABLE (OrderID INT, OrderDate DATETIME)    
DECLARE @OrderDetail TABLE (OrderID INT, ItemID VARCHAR(1), Name VARCHAR(50), Qty INT)    
INSERT @Order VALUES (1, '2010-01-01'), (2, '2010-01-02')    
INSERT @OrderDetail VALUES (1, 'A', 'Drink',  5),
                           (1, 'B', 'Cup',    2),
                           (2, 'A', 'Drink',  2),
                           (2, 'C', 'Straw',  1),
                           (2, 'D', 'Napkin', 1)

-- FOR XML PATH query
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)

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

This solution utilizes a FOR XML EXPLICIT query to wrap the FOR XML PATH query result, effectively removing the unwanted namespace declarations.

1. @xml AS [xml!1!!xmltext]: This directive specifies the XML content to extract from the FOR XML PATH result, excluding the dummy root node.
2. 'http://test.com/order' AS [xml!1!xmlns]: This directive explicitly sets the desired namespace for the outermost node, eliminating duplicate declarations in nested queries.

Result

The output from this query 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>

The namespace declaration is now present only on the root node, as intended.

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