We use the Views UI a lot to create custom pages, blocks, etc. Sometimes it does not give us exactly what we want and we would like to modify the generated sql query. It's possible to achieve this through the 'views_query_alter' hook.
Displaying hierarchical content is a common request of database-driven web sites. Drupal gives you great flexibility in doing this without writing a single line of code, using a couple of battle-tested modules.
