mySQL - updating a qty field from adding and subtracting fields and capture timestamps? How?

I have one table:

I first create a new record in CARTONS_CURRENT inserting values to "part_no" and "qty".

Next, I later need to add or pull (subtract) from "qty" using two other columns for adding and pulling.

How do I make "qty" show the latest result from CARTONS_ADDED and CARTONS_PULLED?

I also need to timestamp each new, add and pull.

 TABLE NAME:  cartons_current
 +------------+--------------+--------+--------+-------------------+------------+
 | Column     |  Type     开发者_如何转开发   |  Null  |  Key   |  Default          |  Extra     |
 +------------+--------------+--------+--------+-------------------+------------+
 | part_no    | varchar(20)  |  No    |  Prim  |                   |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | qty        | int(8)       |  No    |        |                   |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | qty_time   | timestamp    |  No    |        | CURRENT_TIMESTAMP |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | add_qty    | int(8)       |  No    |        |                   |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | add_time   | timestamp    |  No    |        | CURRENT_TIMESTAMP |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | pull_qty   | int(8)       |  No    |        |                   |            |
 +------------+--------------+--------+--------+-------------------+------------+
 | pull_time  | timestamp    |  No    |        | CURRENT_TIMESTAMP |            |
 +------------+--------------+--------+--------+-------------------+------------+

In a BEFORE UPDATE trigger you can change the values, so you can do your timestamp magic in those, you you store the last change.

DELIMITER $$

CREATE TRIGGER ai_cartons_each BEFORE UPDATE ON cartons 
FOR EACH ROW
BEGIN
  IF new.qty_added <> old.qty_added THEN 
    SET new.added_timestamp = NOW();
  END IF;
END $$

DELIMITER ;

See also this answer to your other question:

How do I update a qty field from adding and subtracting fields with timestamps?