year_end < 2014 OR year_start > 2018
NOT(year_end < 2014 OR year_start > 2018)
!(a | b) === (!a) & (!b)
и приведём условие к виду:year_end >= 2014 AND year_start <= 2018
--UPDATE stats SET NEW.globalnum = NEW.num1 + NEW.num2;
++SET NEW.globalnum = NEW.num1 + NEW.num2;