Defining the join type (one to one ) in Join attribute by location (summary) in QGIS

Question

I have two shapefiles. One of them is residential area and the other one is a polygon shapefile which has the population field.

Now I want to do Join attribute by location (summary) analysis but in some areas my second shapefile intersects two polygons from the residential areas shapefile and gives the population to both the residential areas.

I thought maybe it is better the residential area with the larger area have the population.

I have no idea how can I do that. How can I do this?

In this picture the purple is residential area and the black grid is my second shapefile which has the population field. As you can see, some of the grid intersects two different residential area.

Answer 1

You can use this workflow:

1. Join attributes by location with Join type = Take attributes of the feature with largest overlap only. With your purple layer as input layer.
2. Statistics by categories with Field to calculate statistics on the attribute you've joined to the purple polygons, like population. Field with categories should be some unique id on your purple layer

Output will be a geometryless table with a row for each purple polygon with many columns of statistics.

3. If you want, you can join this back to the start layer with Join attributes by field value to get all attributes you started with, the statistics, and the geometries.