Another method is to add a new column, "Year Constructed," using a conditional expression like this:
Case
when "P1939" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1939
when "P1940" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1940
when "P1950" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1950
when "P1960" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1960
when "P1970" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1970
when "P1980" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1980
when "P1990" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 1990
when "P2000" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 2000
when "P2010" = max("P1939", "P1940", "P1950", "P1960", "P1970", "P1980", "P1990", "P2000", "P2010") then 2010
else 0
end
Apply a categorized style using the new "Year Constructed" column.
max
-function in the field calculator (which can also be use as a base for classification)?