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I have a shapefile with points that are joined to the closest twenty points from another shapefile. The joined shapefile has columns from the original shapefile: name (ID), elevation (Z), name from joined shapefile (name) and elevation from the joined shapefile (Z2).

So twenty rows have the same ID and name but differing Z and Z2. I would like to use the field calculator to return the row that has the smallest absolute difference between Z and Z2 with the matching ID and name.

Using a case statement in the qgis field calculator I can get all the results to return the difference in a new column:

CASE WHEN "Name" = "ID" THEN abs("Z" - "Z2") END

but I would like only the smallest value to return.

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    Take a look at minimum expression Commented Dec 19, 2019 at 19:57

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If you would like to return the smallest absolute value, you can take the minimum of each Name group by using the minimum function like so:

case when "Name" = "L1_ID" then minimum(abs("L1_Elev" - "Elev2"), "Name") end
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