如何确定更新的列?

我有这些桌子:

// users
+----+--------+--------------+------------+---------------------------+
| id |  name  |    phone     | postalcode |           email           |
+----+--------+--------------+------------+---------------------------+
| 1  | john   | 0338294214   | 65462345   | fer.74@gmail.com          |
| 2  | jack   | 0657359930   | 93827424   | jack_m@yahoo.com          |
| 3  | peter  | 0083247242   | 99849422   | mv2003@hotmail.com        |
+----+--------+--------------+------------+---------------------------+

// notifications
+----+---------+---------------+------------+
| id | user_id |    content    | timestamp  |
+----+---------+---------------+------------+
| 1  | 2       | phone updated | 1452642595 |
+----+---------+---------------+------------+

我还有这个触发器 更新后 在…上 users 表::

DELIMITER //
CREATE TRIGGER `send_notification` AFTER UPDATE ON `users`
FOR EACH ROW BEGIN
  INSERT INTO notification(user_id, content, timestamp)
                    values(new.id, "phone updated", UNIX_TIMESTAMP())
END
//
DELIMITER 

我所要做的就是在 phone 列更新。(不是在以下情况下 name 或 postalcode 或 email 更新)。如何定义该条件?