[blowski]

I don't remember using recursion in a real project, but I built a HN clone on top of Postgres, with the following query:

    WITH RECURSIVE thread(id, parent_id, user_id, post_id, timestamp, text, depth) AS (
      SELECT id, parent_id, user_id, post_id, timestamp, text, 0
      FROM comments
      WHERE user_id = 1
        AND parent_id IS NULL
      UNION ALL
      SELECT c.id, c.parent_id, c.user_id, c.post_id, c.timestamp, c.text, t.depth + 1
      FROM comments c
      JOIN thread t ON c.parent_id = t.id
      WHERE c.user_id != t.user_id
    )
    SELECT * FROM thread ORDER BY timestamp ASC;

[epgui]

I wrote a recursive CTE to do a tree traversal on a parent-child “relationship” table a few weeks ago at work.

They do come up!