Randomly sample points within polygons

Question

I am working in QGIS 3.24.0. I have 200 vector layers in the form of circular buffers (same size, different extent) and a single layer of points (n=1,000 or so) contained within those buffers. 2 of the buffers are shown below as an example. Each buffer has ~5 points within. I want to select a random point from within each buffer.

How do I accomplish this?

Answer 1

You can achieve this using a virtual layer.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Feel free to add/remove any field of interest from the two layers.

This query makes use of the fact that in SQLlite a group by without an aggregator (min, max etc) will return values of the "first" row. By specifying a random order in the subquery, we effectively get a random point per polygon.

You can then export the virtual layer to a shapefile or else. Note that each time you pan the map the virtual layer is recomputed and therefore a different random point will be selected.

select * 
from
(
 select a.id as poly_id, b.*
 from mypoly_layer a 
  join mypoint_layer b 
   on st_intersects(a.geometry, b.geometry)
 order by random()
 )
group by poly_id