delimiter ;
drop function if exists id2short;
delimiter //
create function id2short (id int)
returns varchar(50)
begin
  set @digits = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
  set @short = substring(@digits, mod(id, 62), 1);
  set id = floor(id / 62);
  while id <> 0 do
    set @short = concat(substring(@digits, mod(id, 62), 1), @short);
    set id = floor(id / 62);
  end while;
  return @short;
end//