drop procedure if exists proc_q80;
delimiter //
CREATE PROCEDURE proc_q80 ()
BEGIN
  declare v_cnt bigint;
  declare v_prev_ts timestamp;
  declare v_ts timestamp;
  declare v_diff bigint;
  declare v_s_diff bigint;
  declare v_done smallint;
  declare c_vendlog CURSOR FOR SELECT sold_ts FROM vending_log order by sold_ts;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_done = 1;

  OPEN c_vendlog;

  set v_prev_ts = null;
  set v_done = 0;
  set v_cnt = 0;
  set v_s_diff = 0;
  REPEAT
    FETCH c_vendlog INTO v_ts;
    IF v_done = 0 THEN
       set v_diff = unix_timestamp(v_ts) - unix_timestamp(v_prev_ts);
       IF v_diff is not null THEN
          set v_cnt = v_cnt + 1;
          set v_s_diff = v_s_diff + v_diff;
       END IF;
       set v_prev_ts = v_ts;
    END IF;
  UNTIL v_done = 1 END REPEAT;
  close c_vendlog;
  SELECT v_s_diff / v_cnt as avg_diff;
END
//
delimiter ;