SQL Server Tree Hierarchy and Nested Sets with Duplicate Record ids

Question

Given that I have this resultset structure (superfluous fields have been stripped)

Id | ParentId | Name | Depth
----------------------------

is it possible to have the records returned in tree order i.e. Parent then Children, if a Child is a Parent, then their Children, if not then Sibling, etc? For example,

Id | ParentId | Name | Depth
----------------------------
1    NULL       Major    1
2    1          Minor    2
3    1          Minor    2
4    3          Build    3
5    3          Build    3
6    1          Minor    2

/* etc, etc */

The only way that I can think of doing this would be to follow this article -

Improve hierarchy performance using nested sets

and include [LeftExtent] and [RightExtent] fields against each record. Now the SQL in the article works fine when Ids are unique, but in this particular tree structure, a record with the same Id can appear in different places within the tree (the ParentId field is different, obviously). I think the problem is in this SQL from the article -

  INSERT INTO @tmpStack
    (
      EmployeeID, 
      LeftExtent
    )
  SELECT TOP 1 EmployeeID, @counter 
  FROM Employee 
  WHERE ISNULL(ParentID, 0) = ISNULL(@parentid,0) 
  /* If the Id has already been added then record is not given [LeftExtent] or [RightExtent] values. */
  AND EmployeeID NOT IN (SELECT EmployeeID FROM @tmpStack)

How can this be altered to allow records with duplicate Ids to be given [LeftExtent] and [RightExtent] values, or I am completely missing an easier way to return the resultset in the order that I require?

Answer 1

You should have a look at recursive common table expressions in SQL Server 2005:

• http://www.4guysfromrolla.com/webtech/071906-1.shtml
• http://www.sqlservercentral.com/articles/Development/recursivequeriesinsqlserver2005/1760/

In your case, this would be something like:

WITH EmployeeCTE AS
(
   -- get the anchor
   SELECT ID, ParentID, Name, 0 as 'Depth'
   FROM Employee WHERE ParentID IS NULL

   -- recursively union lower levels
   UNION ALL
   SELECT e.ID, e.ParentID, e.Name, e.Depth+1
   FROM Employee e
   INNER JOIN EmployeeCTE ON e.ParentID = EmployeeCTE.ID
)
SELECT * FROM EmployeeCTE

This should give you a nice query result set with the data you're looking for. Or am I missing something?

Marc

Answer 2

Here's one that does the trick for me:

@ParentID is just a starting point in the hierarchy, but you can pass in 0 (but I think you're using null as the base ID, so you'll get the idea)

The key to ordered sorting is with the sort key that's built up.

WITH RoleHierarchy (RoleID, [Role], [Description], ParentID, Editable, HierarchyLevel, SortKey) AS
(
   -- Base
   SELECT
     RoleID,
     [Role],
     [Description],
     ParentID,
     Editable,
     0 as HierarchyLevel,
     CAST(RoleID AS VARBINARY(300))
   FROM
     dbo.Roles  
   WHERE
     RoleID = @ParentID

   UNION ALL

   -- Recursive
   SELECT
     e.RoleID,
     e.[Role],
     e.[Description],
     e.ParentID,
     e.Editable,
     th.HierarchyLevel + 1 AS HierarchyLevel,
     CAST (th.SortKey + CAST (e.[Role] AS VARBINARY(100)) AS VARBINARY(300))
   FROM
     Roles e
     INNER JOIN RoleHierarchy th ON e.ParentID = th.RoleID
    WHERE
     e.RoleID != 0
)

SELECT
    RoleID,
    ParentID,
    [Role],
    [Description],
    Editable,
    HierarchyLevel
FROM
    RoleHierarchy
WHERE
    RoleID != @ParentID
ORDER BY
    SortKey

Answer 3

If you used materialized path or HIERARCHYID, you life would be much easier...