Select and display 1-n relations between shape and CSV table

Question

I have a shapefile containing parcels borders (polygons) and a csv table with owner information. I can relate these (1:n) by the parcelnumber. So by identification I can see by clicking on the geometry if there are one or more then one owners of the property. By how can I query all the possible owners by selecting some relevant parcels? and how can I display all possible owners, if there are more than one?

Answer 1

Using aggregate() function, you can relate to another layer and bring together (aggregate) all attribute values and filter them by a condition. So the expression to use on the parcel layer to achieve what you want is:

array_to_string (
    aggregate( 
        'owner', 
        'array_agg', 
        name,
        filter:= parcelnr=attribute (@parent,'parcelnr')
    )
)

If you have overlapping features, like a garage on two or more parcels (your comment), this does not affect the solution, you don't have to change anything, just be sure to make the correct connections in the non spatial table (here: layer owner), see:

By the way: For larger datasets, you might consider an intermediate table, similar to this or this or this, like:

1. Layer parcels (as you have now),
2. Layer owner, containing just the persons,
3. Layer connections, connecting owners to parcels.