Counting points in hex grid by attribute in QGIS

Question

I'm trying to count points within a polygon layer (hexagonal grid), grouped by an attribute of the points. There are some solutions on here (Python console, SQL virtual layer) but I can't seem to get it to work.

An example to illustrate:

Points:

ID	Attribute
1	A
2	B
3	C
4	A
5	B

The output I want for example:

polygon	countA	countB
1	2	1
2	3	13
3	6	2

I've tried this: Count points in polygon grouping by attribute using PyQGIS but the counts come back as 0 everywhere.

Answer 1

Use the following QGIS expression on the polygon layer to get the number of features in layer 'Points' with the value of the field called "attribute" = A:

array_length(overlay_contains('Points', $id, filter:=attribute='A'))

Label generated by concatenating the above expression three times for A, B and C:

Answer 2

There is a possibility of using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

Let's assume there is a point layer called 'Random points in polygons' and a polygon layer called 'municipality_1' with their attribute tables.

With the following query, it is possible to count points within a polygon layer, grouped by an attribute of the points.

SELECT poly.*, countA, countB, countC
FROM "municipality_1" AS poly

LEFT JOIN (
    SELECT poly.id AS idA, count(1) AS countA
    FROM "Random points in polygons" AS poi, "municipality_1" AS poly
    WHERE st_within(poi.geometry, poly.geometry)
      AND poi.attribute = 'A'
    GROUP BY poly.id
    ) ON poly.id = idA

LEFT JOIN (
    SELECT poly.id AS idB, count(1) AS countB
    FROM "Random points in polygons" AS poi, "municipality_1" AS poly
    WHERE st_within(poi.geometry, poly.geometry)
      AND poi.attribute = 'B'
    GROUP BY poly.id
    ) ON poly.id = idB
    
LEFT JOIN (
    SELECT poly.id AS idC, count(1) AS countC
    FROM "Random points in polygons" AS poi, "municipality_1" AS poly
    WHERE st_within(poi.geometry, poly.geometry)
      AND poi.attribute = 'C'
    GROUP BY poly.id
    ) ON poly.id = idC

The output polygon layer with its attribute table will look like this: