merge into some_table OLD
using (  select ? "A", ? "B", ? "C", ? "D" from dual ) NEW
on (OLD.A = NEW.A and OLD.B = NEW.B)
when matched then update set
   calc_date = sysdate,
   C = NEW.C,
   D = NEW.D
when not matched then
  insert(A,B,C,D,calc_date)
  values(NEW.A,NEW.B,NEW.C,NEW.D,sysdate)