QGIS distance between centroids from NUTS1 information

Question

I have calculated the centroids from NUTS1 information. Now for every neighbor centroids, I would like to calculate the distance. I want to connect only the neighbor's Centroids regarding the NUTS1. I tried the formula join attributes by nearest but as the title suggests joins the nearest and not the neighbors.

Here is a screenshot of the problem

Any suggestions?

Furthermore, after tries, I manage to solve the problem. First I created a distance matrix with k nearest equal to 7. Then I duplicate the distance matrix and by using "Join by lines" I connected all the relevant centroids. At this point, I start manually deleting the lines that were not correct according to the neighbor's idea. After that, I checked the attribute table (in order to save in a csv) but there were multiple duplicates for the same connected line. Perhaps the command Join by lines caused the duplicates problem. I export it in excel. Where I could erase easily all the duplicates with a command.

The map looks like that right now

Answer 1

Use "Join Attributes By Location" with the "touches" predicate:

This will return a polygon layer with lots of polygons in it. Here's a chunk of the attribute table:

showing (look at the fid and fid_2 columns) region 1 neighbours regions 2 and 3, and region 10 neighbours regions 9, 22, and 20. So the geometry for region 1 is repeated twice, and the geometry for region 10 is repeated 3 times. Depending on what you want to do with the adjacency you could drop the geometry and have this is a non-geometric table and then join it to you polygons via QGIS 'Join' on the fid.

The X and Y attributes here were the start of an attempt to draw the connecting lines for this neighbourhood network, but I've not done that yet.

Answer 2

You can create a Virtual Layer that will query the polygon layer twice and compute the distance. It is actually simpler to compute the centroids on the fly

go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Feel free to add more fields from both layers.

SELECT a.id, b.id, st_distance(st_centroid(a.geometry),st_centroid(b.geometry)) dist
FROM myPolygonLayer a
  JOIN myPolygonLayer b ON ST_INTERSECTS(a.geometry,b.geometry)
WHERE a.id <> b.id 

The line WHERE a.id <> b.id will returns the same distance twice (from A to B, and from B to A). If you want a single distance, change it to WHERE a.id > b.id 1

If you do need/want to use the already computed centroid layer, you would need to add two more joins to the query. You can add any field from p1 (the source polygon), c1 (the centroid from the source polygon), p2 (the neighbor polygon) and c2(the centroid from the neighbor polygon)

SELECT p1.id, p2.id, st_distance(c1.geometry,c2.geometry) dist
FROM myPolygonLayer p1
 JOIN myPointLayer c1 ON ST_INTERSECTS(p1.geometry,c1.geometry)
 JOIN myPolygonLayer p2 ON ST_INTERSECTS(p1.geometry,p2.geometry)
 JOIN myPointLayer c2 ON ST_INTERSECTS(p2.geometry,c2.geometry)
WHERE c1.id <> c2.id