Mysql 什么触发'；如果语法在下面的程序中是什么意思？

我在理解这个触发语句时有点小问题。你能给我解释一下这句话吗

如果new.id\u类别为（'list'、'of'、'special'、'categories'）

---

语句意思？？？

是否有任何方法允许值的范围匹配？比如new.id\u category=1或new.id\u category=2或new.id\u category=3等等？？？

DELIMITER $$  
create trigger mytable_check before insert on test.mytable for each row  
begin  
 if new.id_category in ('list','of','special','categories')  
    and exists  
      (select * from mytable  
       where id_category=new.id_category  
         and keywords=new.keywords) then  
    call fail(concat('id_category,keywords must be unique when id_category is: ',new.id_category));  
 end if;  
end $$  
DELIMITER ;  

new.id_category = 'list'
or
new.id_category = 'of'
or
new.id_category = 'special'
or
new.id_category = 'categories'