Finding breadcrumbs for nested sets

I'm using nested sets (aka modified preorder tree traversal) to store a list of groups, and I'm trying to find a quick way to generate breadcrumbs (as a string, not a table) for ALL of the groups at once. My data is also stored using the adjacency list model (there are triggers to keep the two in sync).

So for example:

ID   Name    ParentId  Left   Right
0    Node A  0         1      12
1    Node B  0         2      5
2    Node C  1         3      4
3    Node D  0         6      11
4    Node E  3         7      8
5    Node F  4         9      9

Which represents the tree:

Node A
- Node B
  - Node C
- Node D
  - Node E
  - Node F

I would like to be able to have a user-defined function that returns a table:

ID  Breadcrumb
0   Node A
1   Node A > Node B
2   Node A > Node B > Node C
3   Node A > Node D
4   Node A > Node D > Node E
5   Node A > Node D > Node F

To make this slightly more complicated (though it's sort of out of the scope of the question), I also have user restrictions that need to be respected. So for example, if I only have access to id=3, when I run the query I should get:

ID  Breadcrumb
3   Node D
4   Node D > Node E
5   Node D > Node F

I do have a user-defined function that takes a userid as a parameter, and returns a table with the ids of all groups that are valid, so as long as somewhere in the query

WHERE group.id IN (SELECT id FROM dbo.getUserGroups(@userid))

it will work.

I have an existing scalar function that can do this, but it just does not work on any reasonable number of groups (takes >10 seconds on 2000 groups). It takes a groupid and userid as a parameter, and returns a nvarchar. It finds the given groups parents (1 query to grab the left/right values, another to find the parents), restricts the list to the groups the user has access to (using the same WHERE clause as above, so yet another query), and then uses a cursor to go through each group and append it to a string, before finally returning that value.

I need a method to do this that will run quickly (eg. <= 1s), on the fly.

This is on SQL Server 2005.

here's the SQL that worked for me to get the "breadcrumb" path from any point in the tree. Hope it helps.

SELECT ancestor.id, ancestor.title, ancestor.alias 
FROM `categories` child, `categories` ancestor 
WHERE child.lft >= ancestor.lft AND child.lft <= ancestor.rgt 
AND child.id = MY_CURRENT_ID 
ORDER BY ancestor.lft

Kath

Ok. This is for MySQL, not SQL Server 2005. It uses a GROUP_CONCAT with a subquery.

This should return the full breadcrumb as single column.

SELECT 
 (SELECT GROUP_CONCAT(parent.name SEPARATOR ' > ')
 FROM category parent
 WHERE node.Left >= parent.Left
 AND node.Right <= parent.Right
 ORDER BY Left
 ) as breadcrumb
FROM category node
ORDER BY Left

If you can, use a path (or I think I've heard it referred as a lineage) field like:

ID   Name    ParentId  Left   Right   Path
0    Node A  0         1      12      0,
1    Node B  0         2      5       0,1,
2    Node C  1         3      4       0,1,2,
3    Node D  0         6      11      0,3,
4    Node E  3         7      8       0,3,4,
5    Node F  4         9      9       0,3,4,

To get just node D and onward (psuedocode):

path = SELECT Path FROM Nodes WHERE ID = 3
SELECT * FROM Nodes WHERE Path LIKE = path + '%'

What I ended up doing is making a large join that simply ties this table to itself, over and over for every level.

First I populate a table @topLevelGroups with just the 1st level groups (if you only have one root you can skip this step), and then @userGroups with the groups that user can see.

SELECT groupid,
   (level1 
    + CASE WHEN level2 IS NOT NULL THEN ' > ' + level2 ELSE '' END
    + CASE WHEN level3 IS NOT NULL THEN ' > ' + level3 ELSE '' END
   )as [breadcrumb]
FROM (
  SELECT g3.*
    ,g1.name as level1
    ,g2.name as level2
    ,g3.name as level3
  FROM @topLevelGroups g1
  INNER JOIN @userGroups g2 ON g2.parentid = g1.groupid and g2.groupid <> g1.groupid
  INNER JOIN @userGroups g3 ON g3.parentid = g2.groupid 

  UNION

  SELECT g2.*
    ,g1.name as level1
    ,g2.name as level2
    ,NULL as level3
  FROM @topLevelGroups g1 
  INNER JOIN @userGroups g2 ON g2.parentid = g1.groupid and g2.groupid <> g1.groupid

  UNION

  SELECT g1.*
    ,g1.name as level1
    ,NULL as level2
    ,NULL as level3 
  FROM @topLevelGroups g1

) a
ORDER BY [breadcrumb]

This is a pretty big hack, and is obviously limited to a certain number of levels (for my app, there is a reasonable limit I can pick), with the problem that the more levels are supported, it increases the number of joins exponentially, and thus is much slower.

Doing it in code is most certainly easier, but for me that is simply not always an option - there are times when I need this available directly from a SQL query.

I'm accepting this as the answer, since it's what I ended up doing and it may work for other people -- however, if someone can come up with a more efficient method I'll change it to them.

I modified the Statement of Kathy to get breadcrumbs for every element

SELECT
    GROUP_CONCAT(
        ancestor.name
        ORDER BY ancestor.lft ASC
        SEPARATOR ' > '
    ),
    child.*
FROM `categories` child
JOIN `categories` ancestor
ON child.lft >= ancestor.lft
AND child.lft <= ancestor.rgt
GROUP BY child.lft
ORDER BY child.lft

Feel free to add a WHERE condition e.g.