The aggregate function in QGIS was designed to work with 2 separate input vector layers, but we can also make it work with a single layer. Essentially performing spatial queries for features within the layer.
Consider this problem. In a layer containing zip codes, we want to find all neighboring zip codes for every polygon. This can be done very easily by creating a new field with an expression with the aggregate function – using the layer itself as the parent layer.
Below is the Zip Codes shapefile from the City of Seattle Open Data Portal. Our task is to add another field which contains all neighboring zip codes for each feature. Note the layer name is Zip_Codes
Open the Attribute Table for the layer. Note the field name containing the 5-digit zip code is ZIPCODE . Open Field Calculator.
Create a new field with the following expression. The key here is the spatial filter touches($geometry, geometry(@parent)) that finds all polygons from the layer that touch the feature being processed.
aggregate( layer:= 'Zip_Codes', aggregate:='concatenate', expression:=ZIPCODE, concatenator:=', ', filter:=touches($geometry, geometry(@parent)) )
You will now have a new field that has the neighbors zip codes for every feature in the layer!
There are many type of aggregate that you can use. If you want to count numbering zip codes, you can use the count aggregate instead of concatenate. The following expression would calculate the number of neighboring polygons.
aggregate( layer:= 'Zip_Codes', aggregate:='count', expression:=$id, filter:=touches($geometry, geometry(@parent)) )