请问一下下面这段程序
proc sql;
select ID, firstname, count(distinct ID) as t_salary
from sasuser.pilots
group by ID
order by firstname;
quit;
为什么有group by又有order by呢?是按这两个变量排序吗?
那为什么不放在一起,写成group by ID firstname或者
order by ID firstname呢?谢谢!