overpass: stats per value

Question

I'd like to generate some statistics about the members of a relation. This is as far as I've gotten:

[out:csv(number,length)][timeout:25];
(
  relation(id:3121667)->.groute;
);
way(r.groute);
make stat number=count(ways),length=sum(length());
out;

Now, I would like to have a row in the CSV for every unique combination of highway and surface. So for example the total length of secondary - asphalt roads in this relation. I looked at the API doc and the examples, but I really don't see how

Accepted Answer

[out:csv(highway,surface,number,length)][timeout:25];
relation(id:3121667)->.groute;
way(r.groute);

for->.hw (t["highway"]) {
  for.hw (t["surface"]) {
    make stat highway=hw.val, surface=_.val, number=count(ways), length=sum(length());
    out;
  }
}

This was a great help to figure out for loops and how to access the tags we're grouping by (hw.val and _.val): https://dev.overpass-api.de/blog/loop_and_group.html

overpass: stats per value

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