[castorp]

> I suspect that's a pretty non-standard/rarely-used feature though. If you learn SQL you likely won't encounter this

Recursive common table expressions are part of the SQL standard (since 1999) and are quite frequently used to traverse hierarchical data (aka "adjacency list").

It is part of basically all (good) SQL tutorials - at least in the "advanced" part.

[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!