How do I remove redundant namespace in nested query when using FOR XML PATH

Question

When using FOR XML PATH and WITH XMLNAMESPACES to declare a default namespace, I will get the namespace decleration duplicated in any top level nodes for nested queries that use FOR XML, I've stumbled across a few solutions on-line, but I'm not totally convinced...

Here's an Complete Example

/*
drop table t1
drop table t2
*/
create table t1 ( c1 int, c2 varchar(50))
create table t2 ( c1 int, c2 int, c3 varchar(50))
insert t1 values 
(1, 'Mouse'),
(2, 'Chicken'),
(3, 'Snake');
insert t2 values
(1, 1, 'Front Right'),
(2, 1, 'Front Left'),
(3, 1, 'Back Right'),
(4, 1, 'Back Left'),
(5, 2, 'Right'),
(6, 2, 'Left')



;with XmlNamespaces( default 'uri:animal')
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')

What's the best solution?

Answer 1

If I have understood correctly, you are referring to the behavior that you might see in a query like this:

DECLARE @Order TABLE (
  OrderID INT, 
  OrderDate DATETIME)

DECLARE @OrderDetail TABLE (
  OrderID INT, 
  ItemID VARCHAR(1), 
  ItemName 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)

;WITH XMLNAMESPACES('http://test.com/order' AS od) 
SELECT
  OrderID AS "@OrderID",
  (SELECT 
     ItemID AS "@od:ItemID", 
     ItemName AS "data()" 
   FROM @OrderDetail 
   WHERE OrderID = o.OrderID 
   FOR XML PATH ('od.Item'), TYPE)
FROM @Order o 
FOR XML PATH ('od.Order'), TYPE, ROOT('xml')

Which gives the following results:

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

As you said, the namespace is repeated in the results of the subqueries.

This behavior is a feature according to this conversation although there is the option to vote on changing it.

My proposed solution is to revert back to FOR XML EXPLICIT:

SELECT
  1 AS Tag,
  NULL AS Parent,
  'http://test.com/order' AS [xml!1!xmlns:od],
  NULL AS [od:Order!2],
  NULL AS [od:Order!2!OrderID],
  NULL AS [od:Item!3],
  NULL AS [od:Item!3!ItemID]
UNION ALL
SELECT 
  2 AS Tag,
  1 AS Parent,
  'http://test.com/order' AS [xml!1!xmlns:od],
  NULL AS [od:Order!2],
  OrderID AS [od:Order!2!OrderID],
  NULL AS [od:Item!3],
  NULL [od:Item!3!ItemID]
FROM @Order 
UNION ALL
SELECT
  3 AS Tag,
  2 AS Parent,
  'http://test.com/order' AS [xml!1!xmlns:od],
  NULL AS [od:Order!2],
  o.OrderID AS [od:Order!2!OrderID],
  d.ItemName AS [od:Item!3],
  d.ItemID AS [od:Item!3!ItemID]
FROM @Order o INNER JOIN @OrderDetail d ON o.OrderID = d.OrderID
ORDER BY [od:Order!2!OrderID], [od:Item!3!ItemID]
FOR XML EXPLICIT

And see these results:

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

The original idea for creating the namespace in FOR XML EXPLICIT comes from here.

Answer 2

An alternative solution I've seen is to add the XMLNAMESPACES declaration after building the xml into a temporary variable:

declare @xml as xml;
select @xml = (
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'))

;with XmlNamespaces( 'uri:animal' as an)
select @xml for xml path('') , root('zoo');