News for hackers. Stuff that matters.

A comment on Things I wished more developers knew about databases. Parent comment

Re:Things I wished more developers knew about databases by karmakaze on Wednesday April 22, 2020 @09:21PM (#22951905)
I don't see how JOIN is like flatMap. It's a subset of a cross-product. It's more like do-notation:

  x <- ..
  y <- ..
  return (x, y)
[ Parent | on HN ]
Re:Things I wished more developers knew about databases by taffer on Thursday April 23, 2020 @04:15AM (#22953825)
> I don't see how JOIN is like flatMap

This is explained in the blog post I linked, including an example.

[ Parent | on HN ]
Re:Things I wished more developers knew about databases by karmakaze on Thursday April 23, 2020 @03:38PM (#22960229)
It's really flatMap/map from the link:

  List<Integer> s1 = Stream.of(1, 2);
  Supplier<Stream<String>> s2 = ()->Stream.of("A", "B");

  s1.flatMap(v1 -> s2.get()
                   .map(v2 -> tuple(v1, v2)))
    .forEach(System.out::println);
That's just a nested loop.
[ Parent | on HN ]