Hacker News Reader

Comment by marcosdumay | Hacker News Reader

SQL is on the same group as Prolog. In fact, it is a simpler and purer implementation of a constraint-solving language, so it is probably a better option for learning the group than Prolog.

For the signal transformation ones, there are also the hardware definition languages on the same category as LabView and animation languages.

View on HN

To your point, Berkeley's intro-level CS class, CS61A, used to use teach Prolog as an example of logic programming. They use SQL now instead.

I can't really attest to whether that's a better choice for learning logic programming, though. It's easier to "run programs" in Prolog. SQL wants to be a cog in a machine where Prolog is more freestanding.

I definitely did a bunch of Prolog in my CMPT216 class back in undergrad and I hadn't really thought of SQL as an alternative until today. I agree with you it's easier to "run programs" in Prolog, and a lot of the concepts from the class would have been more difficult to map to SQL than Prolog (e.g. writing a Sudoku solver in Prolog was mind-melting in a good way, writing a Sudoku solver in SQL would probably be mind-melting in a bad way). The one thing that SQL does have going for it though is that there's a ton of tooling for getting practical real-world data into it.

I would absolutely love to take a set of logic/set-oriented problems and solve them both in Prolog and SQL, just to see which ones are ergonomic in each language. Maybe this summer at the cabin...

The tooling thing goes both ways. I remember classmates struggling with homework because they were trying to use a different flavor of SQL than the SQLite we ran for class. This stuff is obvious to engineers, but very confusing to someone whose first hello-world was 8 weeks ago.

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.

Yes it does. It’s a fun problem to make a sudoku solver

https://technology.amis.nl/it/solving-a-sudoku-with-one-sql-...

Ok fair enough. I suspect that's a pretty non-standard/rarely-used feature though. If you learn SQL you likely won't encounter this and I still contest the idea that SQL is a good entry into the logic paradigm.

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.

> 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.

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;

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!

Oh, it's certainly not a "type specimen". It does actually have recursion and matching-like behavior, but those are convoluted and not really used, so it is not very suitable for general programming.

But lacking those general features makes it an even purer "learning example" of the group.

Reply on news.ycombinator.com