Counting number of features within another feature using aggregate function in the QGIS Field calculator

Question

My data: 1 polygon layer + 1 point layer
My target: Calculate the number of points and the sum of a field within each polygon using the aggregate function in the QGIS field calculator.

I know I can simply use the QGIS functions "Join attributes by location (summary)" or "Count points in polygon" but I'm interested in the aggregate function in the field calculator since I don ́t understand exactly its meaning.

Using

aggregate('hh', 'sum', "sum_pop")

'hh' - the name of the point layer
'sum_pop' - the name of the attribute field from the hh layer (sum of the population of a point)

I get the sum of the whole field in the attribute table, so each feature has the same value.

aggregate_result_nok

How can I change the code for the aggregate function to calculate the sum of a value based on the number of points in each polygon feature and the number of features within a polygon so I get different values for each polygon?

example1

ecample2

evaluation error

• csk: You need to add a group_by parameter. Does the point layer have a polygon_ID field that tells you which polygon each point belongs to? If so, use "polygon_ID" as the group_by parameter.
• Mapos: @csk The point layer does not have a field with the polygon affiliation. But using the function Join attributes by location this field can be generated.

Answer 1

This expression will count the number of populated places for each country:

aggregate(layer:='ne_110m_populated_places',
 aggregate:='count',
 expression:=name,
 filter:=contains(geometry(@parent), $geometry)
 )

you can use the same expression to get the sum, just change the aggregate to 'sum' and the expression to the field name to summarize.

• Mapos: Thank you for your quick response. For point counts the expression functions well (the expression parameter as string needs to be in parentheses). Although I don´t exactly understand the meaning of this expression parameter (in this case "sub expression") since I can write there whatever I want but ok. For 'sum' I just get NULL values (changing the expression parameter to "pop_sum" as the field name) or an error (see above). pop_sum field is integer.
• Mapos: aggregate(layer:= 'hh', aggregate:= 'sum', expression:= 'sum_pop_in', filter:= contains(geometry(@parent), $geometry))
• eurojam: the expression should be without any quotation marks: expression:= sum_pop_in

Answer 2

Since QGIS 3.16 (see the Changelog) there is also a possibility using the overlay_contains() function

With the following expression, it is possible to count the number of features within another feature (see example below)

array_length(overlay_contains('random_points_test', expression:=1))

example