为不同的操作创建两个触发器
用于更新操作

create trigger update_t1
after update  on t1 for each row
begin

select count(*) from t2 where id=New.id  into @num;

if @num=0 then
/*insert into t2; */

else
/*update t2 */
end if;
end;

用于插入操作

create trigger insert_t1
    after insert  on t1 for each row
    begin

    /*insert into t2; */

    end;