Finding breadcrumbs for nested sets

Question

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

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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.

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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.

Answer 1

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 + '%'

Answer 2

Hi Gregmac,

no sql server specific code, but are you simply looking for :

SELECT * FROM table WHERE left < (currentid.left) AND right > (currentid.right)

Answer 3

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.

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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.