Placeholder item in overpass API

Question

Say I have a simple query that may or may not give a result. (and only one result, say with out 1) It runs fast, so I bundle them together into a single query. But the problem is that sometimes it will not return a result, so is there a simple solution to this? It would be easy if I there was a placeholder item in OSM, like this:

query goes here
way(0);

So that when processing the output, you could detect the placeholder.

Is there any nice way to do this?

Accepted Answer

First of all, a quick and dirty approach similar to your post.

node(1);out ids;

This will add a one liner to your query result, regardless of whether there was some result or not.

  <node id="1"/>

It shouldn't add too much overhead, after all.

There's one potential issue with that approach, though: if some mapper happens to delete that node #1, your marker is gone and you'll have to find some other available node. Of course, your normal query result also shouldn't produce a result, which looks like the marker, otherwise you cannot tell the both of them apart any longer.

The recommended approach involves the use of out count;. In the following example, we will first print all the buildings in the current bbox, followed by a total count:

[bbox:{{bbox}}];
way[building];
out geom;
out count;

Assuming there are no buildings in your bbox, you will still get the following summary information:

  <count total="0" nodes="0" ways="0" relations="0" areas="0"/>

Placeholder item in overpass API

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