A hierarchical query 

 comment_id | thread_id | parent | score |                  text
------------+-----------+--------+-------+---------------------------------------
          3 |         2 |        |     0 | Crap comment that nobody wants to read
          4 |         2 |        |    19 | Top comment
          7 |         2 |        |     3 | Less highly rated comment
          8 |         2 |      4 |    22 | Top reply to top comment
          5 |         2 |      4 |     6 | Earliest reply to top comment
         10 |         2 |      7 |     5 | Reply to less highly rated comment
         11 |         2 |      8 |    12 | Top response to top reply
          9 |         2 |      8 |     1 | Meh response to top reply

• Equivalent to:

SELECT * FROM comment WHERE thread_id = 2;