Question

I have a table which defines a child-parent relationship between nodes:

CREATE TABLE node (                           ' pseudo code alert   id        INTEGER PRIMARY KEY,   parentID  INTEGER, ' should be a valid id. ) 

If parentID always points to a valid existing node, then this will naturally define a tree structure.

If the parentID is NULL then we may assume that the node is a root node.

How would I:

1. Find all the nodes which are decendents of a given node?
2. Find all the nodes under a given node to a specific depth?

I would like to do each of these as a single SQL (I expect it would necessarily be recursive) or two mutually recursive queries.

I’m doing this in an ODBC context, so I can’t rely on any vendor specific features.

Edit

• No tables are written yet, so adding extra columns/tables is perfectly acceptable.
• The tree will potentially be updated and added to quite often; auxillary data structures/tables/columns would be possible, though need to be kept up-to-date. If you have any magic books you reach for for this kind of query, I’d like to know.

Many thanks.

Answer 1

This link provides a tutorial on both the Adjacency List Model (as described in the question), and the Nested Set Model. It is written as part of the documentation for MySQL.

What is not discussed in that article is insertion/delection time, and maintenance cost of the two approaches. For example:

• a dynamically grown tree using the Nested Set Model would seem to need some maintenance to maintain the nesting (e.g. renumbering all left and right set numbers)
• removal of a node in the adjacency list model would require updates in at least one other row.