Copy in adjacency model

Question

I need to create a sql stored procedure (Sql Server 2008 - T-SQL) which copies a node in an adjacency model.

Table can be seen as having two columns, Id and ParentId (FK to Id). Copying means that also all subordinates need to be copied.

I think that using WITH is a good start, but I'm curious if I can do this copy without using Cursors.

Answer 1

The fundamental problem with adjacency lists is there is no general way in SQL to extract an entire sub tree, so you already have a problem of identifying all the rows you need to duplicate without resorting to a cursor.

If possible migrate your adjacency list to a nested set model which allows you to easily identify all the nodes of a subtree. However, the maintenance of a nested set model is more complex for general inserts and deletes.

EDIT: As pointed out by 'a_horse_with_no_name' there is a way in general SQL to process adjacency lists, recursive common table expressions.

Answer 2

Copying a whole sub-tree is a bit of a problem because when you copy your sub-tree you are either

• denormalizing data or
• using it as a template of some sorts.

In either case you are dragging data through inconsistent state at some point - which indicates some problems with your design (for example do your records need to have multiple parents or not? if yes, then you should consider redesigning).

So, you should update the answer with a more complete example of what you are trying to do.

One solution would be to have a temporary table, selecting for the insert should not be a problem, it is just updating the referenced IDs that would be a problem.

So

1. WITH INSERT into temporary table
2. UPDATE the IDs
3. INSERT into original table
4. DELETE temp records

The procedure needs to go like this because it would be hard to change the IDs (both record IDs and ID referring to parent) in initial WITH INSERT. However it might be possible, if there was a nice function that depended only on max_id or only on old IDs.