MYSQL触发器的问题，请高手解答

fetchpaper表中有num字段，添加触发器的目的就是让num自动填入到数据库中，利用触发器自动增加并且不能重复，这是我根据oracle数据库触发器写的，大侠们看看错再哪里？
CREATE TRIGGER TIB_FETCHPAPER BEFORE
INSERT ON FETCHPAPER FOR EACH ROW
declare @row_count int(10),
declare @max_num char(100),
declare @zero_today char(100),
begin
case
when inserting then
@zero_today =select CONCAT('0391',DATE_FORMAT(curdate(),'%Y%m%d'),RPAD('0',4,'0'))
  select @row_count=count(*) from fetchpaper
  if @row_count = 0 then
        @max_num=@zero_today
    else
      select @max_num=max(num)from fetchpaper
    end if
    if @max_num <@zero_today then
        @max_num=@zero_today
    end if
if NEW.num is null then
    NEW.num=select rpad((@max_num)+1),14,'0')
    end if
  when updating('num') then
  if NEW.num!=OLD.num then
      NEW.num=OLD.num
  end if
end case
end;

---

create table FETCHPAPER
(
  ID                  int not null auto_increment comment '实名制领料单ID',
  WAREHOUSEID          int