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I would like to filter a layer with the maximum value of field_a, grouped by field_b. I tried this expression: "field_a"= maximum ( "field_a","field_b") But it doesn't work (there are no more features in the layer). When I use this expression for Select by Expression, the features are selected. Is there a way to filter them (without selecting)?
[Edit] I want to filter a existing layer (right mouse click -> filter). I don't want to select features or to create a new layer.
The filter query (layer properties/source/provider feature filter) allows you to specify an SQL where clause. If you enter id=1 it will translate it to select * from mylayer where id=1.
From there, you can add a sub-query in this where clause to identify the IDs (or else) of interest:
field_a IN (SELECT MAX(field_a) FROM myLayer GROUP BY field_b)
If this is for visualization purpose, I suggest Rule-based symbology.
"field_a" = maximum("field_a", group_by:= "field_b" )
[EDIT] To actually reduce the features by the expression, try Extract by expression tool (in Processing toolbox > Vector selection).
The expression is the same as the above.
