SQL查询疑惑

有个疑惑，写法一是我自己写的，二和三是作者给出，有人懂得为什么不同吗。 --写法一 select ename,sal,job,comm,is_salesman from (select ename,sal,job,comm, case when job = 'SALESMAN' then 1 else 0 end is_salesman from emp) order by comm asc,is_salesman desc; --写法二 select ename,sal,job,comm from emp order by case when job='SALESMAN' then comm else sal end; --写法三 select ename,sal,job,comm, case when job='SALESMAN' then comm else sal end ordered from emp order by 5;