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I've got a colleague who wants to create a (concatenated) label based on a 1:n relation between some shapes and related tables. They asked me if this was possible in QGIS - and I think it should.

I found this: QGIS: relations in label expressions

However, I failed to generalise this for 1:n and m:n relations. The solution provided by @ben-w isn't using the relation, and the solution by @alexander-novikov uses relation_aggregate() which is not really suitable for simply just getting multiple character values our of a relation.

Can someone point me to to right direction, and maybe provide some syntax explanation?

(NB: Sorry not to provide a reproducible example. I not using QGIS expressions normally, I am coming from R using {dplyr}, and I don't speak SQL, nor Python. Different syntax, maybe sometimes different logic. R is no solution for my colleague, so I try to learn something new.)

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You can use the aggregate function in the expression editor:

 aggregate( layer:= 'N_LAYER',
            aggregate:='concatenate', 
            expression:="FIELDNAME_IN_N_LAYER",    
            concatenator:=', ',
            filter:=attribute(  $currentfeature ,'FOREIGNKEY_IN_N_LAYER')=attribute(  @parent ,'KEY_IN_1_LAYER'))

where you tell the aggregate function in the filter criteria the relation between the key/foreign key. Be aware that $currentfeature belongs to the n_layer and @parent to the 1_layer in your 1:n relation.

  • Thanks. Took me a while to parse this. expression := gets whatever field (or even concat("attribute_field1", ', ' "attibute_field2"), concatenator := sets the separator for the 1:n relations, i.e. multiple matches. The filter := part is what I was looking for. I think I could not have learned this from docs.qgis.org/testing/en/docs/user_manual/working_with_vector/… (and I'm not yet sure how this, um, relates to relation_aggregate. I'm going try to make this work for n:1 and m:n relations as well. Your help is much appreciated. :) – R'n'E Apr 29 at 12:35

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