I have a point layer, consisting of 12 points with fid 1 to 12. I want to render points with fid > median (fid) - thus in this case, point 7 to 12. That works perfectly if using geometry generator with this expression - see screenshot, where only 7 to 12 are rendered, whereas 1 to 6 only have labels/callouts:

if ( 
    "point_id" > median (
    $geometry , 

enter image description here

However, when I use the same expression with Menu Processing / Toolbox /Geometry by expression on the same point layer, I get an error: Cannot use relation aggregate function in this context. Why is that - or asked the other way round: how to correct the expression? It's the same expression as in geometry generator in rather the same context.

I am asking specifically about this relation aggregate function error, as I already encountered it several times in more complex cases. In this simple case, of course there would be easier ways to achieve the goal. However, I wanted to present a simple case to make the error easier to reproduce.

1 Answer 1


When using aggregate functions with processing it seems there is a scope problem.

For your problem, you should use the aggregate function and refer to the layername of your source point layer.

Something like that :

if (

    "point_id" > aggregate('<layername>','median', "point_id"), 
    $geometry , 
    geom_from_wkt('POINT EMPTY')

You need also to create a valid geometry point with geom_from_wkt('POINT EMPTY')

  • OK, works perfectly! It really seems that not only aggregate functions behave different in geometry generator and Geometry by expression tool, but empty outputs as well: in Geometry by expression, indeed only geom_from_wkt('POINT EMPTY') works, whereas in geometry generator, an empty output '' works well.
    – Babel
    Commented Jan 18, 2021 at 20:53

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