CTE and FOR XML to generate nested XML

Question

I have an adjacency list in the DB and want to deliver the data in XML format to the client through a SQL SP. I\'m trying to use CTE and FOR XML but I am not getting the XM

Answer 1

Recursive CTEs are not recursive as in "nested", they work differently and what you're trying to do doesn't work with CTEs. (Think of them as being always tail-recursive.)

The only way I have found to build recursive XML in SQL Server is by creating a scalar function which renders the nodes recursively; functions can make recursive calls so that this works as expected.

Answer 2

The question as well as the OP's answer helped me a lot. It took me a bit to grasp the answer as I was missing some context. So here's a seperate answer with a more generic explanation (I've tried to remove every bit of code that didn't relate directly to getting hierarchical data in XML output).

---

Suppose the following typical table for hierarchical data:

CREATE TABLE Employee (Id INT, BossId INT, Name NVARCHAR(50));

Suppose it has the following data:

INSERT INTO Employee (Id, BossId, Name) VALUES
(1, NULL, 'Boss Pancone'),
(2, 1, 'Capioregime Luciano'),
(3, 1, 'Capioregime Bruno'),
(4, 2, 'Johnny'),
(5, 2, 'Luca'),
(6, 2, 'Luciano jr.'),
(7, 3, 'Marco'),
(8, 3, 'Mario'),
(9, 3, 'Giacomo');

To get hierarchical XML data we could use the following function:

ALTER FUNCTION dbo.fn_EmployeeHierarchyNode (@BossId INT) RETURNS XML
BEGIN RETURN
    (SELECT Id, 
            BossId, 
            Name,
            dbo.fn_EmployeeHierarchyNode(Id)
        FROM Employee
        WHERE BossId = @BossId
        FOR XML AUTO)
END;

Which can be called like this:

SELECT dbo.fn_EmployeeHierarchyNode(1)

Or, if you want the root node as well, like this:

SELECT  Id,
        BossId,
        Name,
        dbo.fn_EmployeeHierarchyNode(Id)
FROM    Employee
WHERE   BossId IS NULL
FOR     XML AUTO

Which would produce:

<Employee Id="1" Name="Boss Pancone">
  <Employee Id="2" BossId="1" Name="Capioregime Luciano">
    <Employee Id="4" BossId="2" Name="Johnny" />
    <Employee Id="5" BossId="2" Name="Luca" />
    <Employee Id="6" BossId="2" Name="Luciano jr." />
  </Employee>
  <Employee Id="3" BossId="1" Name="Capioregime Bruno">
    <Employee Id="7" BossId="3" Name="Marco" />
    <Employee Id="8" BossId="3" Name="Mario" />
    <Employee Id="9" BossId="3" Name="Giacomo" />
  </Employee>
</Employee>

Answer 3

Turns out I didn't want the CTE at all, just a UDF that I call recursively

CREATE FUNCTION PageHierarchyNode(@PageId int)
RETURNS XML
WITH RETURNS NULL ON NULL INPUT 
BEGIN RETURN 
  (SELECT ModuleId AS "@ModuleId", PageId AS "@PageId",
    ParentPageId AS "@ParentPageId", PageUrl AS "@PageUrl",
    PageTitle AS "@PageTitle", PageOrder AS "@PageOrder", 
      CASE WHEN ParentPageId=@PageId
      THEN dbo.PageHierarchyNode(PageId)
      END
   FROM dbo.PageHierarchy WHERE ParentPageId=@PageId
   FOR XML PATH('Page'), TYPE)
END

with the SQL that calls the UDF as

SELECT ModuleId AS "@ModuleId", PageId AS "@PageId",
    ParentPageId AS "@ParentPageId", PageUrl AS "@PageUrl",
    PageTitle AS "@PageTitle", PageOrder AS "@PageOrder", 
    dbo.PageHierarchyNode(PageId)
FROM PageHierarchy
WHERE ParentPageId IS NULL
FOR XML PATH('Page'), ROOT('SiteMap'), TYPE