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I want to add columns in a shapefile to make calculations on it, those calculations are supposed to measure the size per land cover type and therefore I need that my calculation adapts to my filters.

I do my filters in the query builder because it does not work from the attribute table.

When I filter my layers the results of my calculation don't change. I don't understand why and I don't know how to solve this issue.

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Using a regular field calculation, the result is static and would need to be re-calculated each time a change is made. To make the output dynamic, you can create a virtual field that will adapt to a filter.

Here I make a field called adaptive that is the sum of another field length

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With no filter applied on the layer:

enter image description here

With the filter fid > 1:

enter image description here

The virtual field persists when you save, close, and re-open your project. However, it does not exist in the actual shapefile, so you would need to export the layer as a new shapefile if you want to keep a static version of your result (to hand to someone else, for example).

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    Thank you, I didn't know about these, or that there was a Field Calculator button in the Attribute Table that works slightly differently from the one in the Toolbox, like offering the Create Virtual Field option. This shouldn't have surprised me, since I came to QGIS from ArcMap, which does the same thing.
    – Cowirrie
    Feb 12, 2022 at 10:31

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