SQL has no recursion. I would argue that's a pretty fundamental aspect of Prolog, so I don't think SQL is a representative replacement.
https://technology.amis.nl/it/solving-a-sudoku-with-one-sql-...
SQL and Prolog are both relational. That's very unique to both. But SQL is all about querying databases. Prolog can also be used and understood as a database querying language but it's also very strong for
- parsers
- interpreters
- expert systems
- solving combinatorial problems
If you really want to you can probably use SQL for that too. Or any language for that matter. But going out and learning SQL won't naturally expose you to these applications and how well the logic paradigm lends itself to them.
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.
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;They do come up!
But lacking those general features makes it an even purer "learning example" of the group.