Summing Area of polygons within vector layer within zone using QGIS?

Question

I have one vector layer with multiple polygons (purple). I also have a number of other vector layers which are catchment zones (I also have a layer where these catchments are merged).

Is there any way I can sum/calculate the areas of the purple polygons within each of the individual catchment zones?

I need this to work out a percentage of the area taken up by the purple polygons within each catchment.

Answer 1

1. Intersect
2. Dissolve based on catchment ID
3. Create new field and calculate area

Answer 2

Tested on QGIS 2.18 and QGIS 3.4

I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer...

Let's assume we have ten features in "multiple_polygons" (yellow), and three in "catchment_zones" (grey) accordingly, see image below.

With the following Query, it is possible to achieve the result

SELECT c.geometry, c.id, c.Name,
       ROUND(st_area(c.geometry),2) AS "catch_area(m*m)",
       ROUND(SUM(st_area(m.geometry)),2) AS "mult_area(m*m)",
       COUNT(m.geometry) AS "num_mult",
       ROUND(SUM(st_area(m.geometry))/st_area(c.geometry)*100,2) AS "area_%"
FROM "catchment_zones" AS c
JOIN "multiple_polygons" AS m ON st_intersects(c.geometry, m.geometry)
GROUP BY c.id

The output Virtual Layer will maintain the original geometry and some initial attributes including several new, namely 'catch_area(mxm)' (in square meters), 'mult_area(mxm)' (in square meters), 'num_mult' and 'area_%'.

P.S. mind the CRS