处理排序空值

SQL> select ename,sal,comm from emp order by 3 desc;
ENAME             SAL       COMM
---------- ---------- ----------
SMITH             800
CLARK            2450
FORD             3000
JAMES             950
ADAMS            1100
JONES            2975
BLAKE            2850
MILLER           1300
SCOTT            3000
KING             5000
MARTIN           1250       1400
WARD             1250        500
ALLEN            1600        300
TURNER           1500          0
14 rows selected.
/* 如上显示，降序排列后奖金显示到了最后面,空值在上，这里需要把空值显示到下面*/
-- 可以使用nulls last函数处理，如果使用nulls first则和上面默认排序结果一样
SQL> select ename,sal,comm from emp order by 3  desc nulls last;
ENAME             SAL       COMM
---------- ---------- ----------
MARTIN           1250       1400
WARD             1250        500
ALLEN            1600        300
TURNER           1500          0
SCOTT            3000
KING             5000
ADAMS            1100
JAMES             950
FORD             3000
MILLER           1300
BLAKE            2850
JONES            2975
SMITH             800
CLARK            2450
14 rows selected.