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;