a question about 'nested sets'

[eluser]WanWizard[/eluser]
Nested sets are about hierarchical data structures, or tree structures.

All you need is a table with an (autoincrement) id column, and left- and right pointer columns. These tree columns define the tree structure. What other fields you add to the table is up to you.

To implement such a structure, simply add any reply as last child of the message id being replied to.