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There is an easy way that needs two steps from the db-manager and two tables that can be deleted in the end(there is a way with virtual tables, but i don´t get them to work properly from the db-manager). go to db-manager and then to virtual layers and select your table with the trees; then start the sql-window Enter into the query field: SELECT ...
With the new virtual layer you can now use SQL on any layer in QGIS, for example the airport Shapefile from the QGIS sample data: To count the number of airports by USE type and compute the average elevation use: The results are loaded as a new layer:
You could to use the "Field Calculator". Try this expression for update your field "group": CASE WHEN "species"='Ash' THEN 1 WHEN "species"='Beech' THEN 2 ...... ...... END
Make a set of unique geometries by deleting duplicates (following the example you have already found - create as a new table). In the new table, create an attribute to uniquely identify each polygon (you could do this as part of step one by adding a serial field). Use the new table to do a spatial join on the original table. this will add the new unique ...
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