I have two polygon shapefiles in QGIS.

I would like to have the polygon "ID"s of 'B' listed in the attribute table of 'A' for the case that they overlap.

As a demonstration:

intersect of two shapefiles

I tried combinations of spatial queries, intersects and spatial joins but I won't find an answer. Spatial join only allows statistical summaries of attributes (sum, average, mean).

I also tried using the model builder but didn't find the right arrangement of tools.


3 Answers 3


A Virtual layer would work well here:

virtual layer dialog with query

resulting table

First hit 'Import' to add your layers then add the query:

SELECT A.id, group_concat(b.id)
LEFT JOIN B ON ST_Intersects(a.geometry, b.geometry)

group_concat() will concatenate all results into a comma separated string (as your above screenshot). If you want to separate by something other than a comma you can define a custom separator.

If you want to name your columns you can use 'AS ...' for instance:

SELECT A.id AS a_id, group_concat(b.id) AS b_intersect ...
  • Thank you. That is, among the current answers, the one that fits to my case best. I work with qgis quite long but didn't know this function.
    – AndyB
    Jan 30, 2017 at 7:18
  • @AndyB I'm glad it worked for you! I just recently learned how to use virtual layers myself. Useful when you want some of the advantages of a database without actually implementing one. Remember to accept the answer if it worked for you. ;)
    – J M
    Jan 30, 2017 at 8:44

Intersect is the right tool. Not sure what happened when you had tested it.

Polygons 'A' and 'B' with each attribute, "A_id" and "B_id".

enter image description here

Intersected output "test_intersect" and its attribute table (RHS).

enter image description here

  • thank you. Unfortunately, it doesn't fully meet my requirements, because I need the IDs of A to remain unique instead of split-up.
    – AndyB
    Jan 30, 2017 at 7:19
  • @AndyB Thanks for your reply. I now understand how intersect alone did not meet your expectation, especially so compared to Jesse's elegant solution. :)
    – Kazuhito
    Jan 30, 2017 at 7:27

Here's an approach that would use the aggregate function in R to get your final data output.

  1. Run an intersection of the two layers. This will result in a layer with each and every combination of intersecting polygons.
  2. Export the attribute data only and load it in R as a data.frame.
  3. Then using the aggregate function to group by the larger polygon ID values (using the c function to concatenate all the smaller polygon IDs to a single row output). E.g., if your data.frame object is called dt then: aggregate(polyIDs1 ~ polyIDs2, data=dt, FUN=c)

Heres a good description

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.