How do you sort a tree stored using the nested set model?

Question

When I refer to nested set model I mean what is described here.

I need to build a new system for storing \"categories\" (I can\'t think of better word for it) in a user

Answer 1

I believe that, in your case, where the nodes you want to swap don't have any descendants, you can simply swap the lft and rgt values around. Consider this tree:

   A
 /   \
B     C
     / \
    D   E

This could turn into this group of nested sets:

1 A 10 
2 B 3  
4 C 9
5 D 6
7 E 8

Now consider you want to swap D and E. The following nested sets are valid and D and E are swapped:

1 A 10
2 B 3 
4 C 9 
7 D 8
5 E 6 

Swapping nodes that have subtrees cannot be done this way, of course, because you would need to update the childrens' lft and rgt values as well.