如何在触发器中放置IF条件以计算列的值

CREATE TABLE IF NOT EXISTS `test` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ia_att` decimal(3,2) NULL,
  `t1` decimal(3,2) NULL,
  `t2` decimal(3,2) NULL,
  `t3` decimal(3,2) NULL,
  `ia_tot` decimal(3,2) NULL,
  .....
  .....
  PRIMARY KEY (`id`),
);   
CREATE TABLE IF NOT EXISTS `test` (
  `id` int(