Getting hierarchy data from self-referencing tables

Question

Let\'s say you have the following table:

items(item_id, item_parent)  

... and it is a self-referencing table - item_parent r

Answer 1

Oracle has a very convenient syntax for retrieving hierarchical data like this:

select
    item_id,
    item_parent,
    level as depth
from
    items
connect by
    prior item_id = item_parent
start with
    item_parent not in (select item_id from items)

This starts with the root nodes of your trees as those items whose item_parent does not exist in the table as item_id, and selects all children of those nodes, along with their depth in the tree.