请教一个SQL语句写法?

Question

环境:oracle
表test,结构如下:
ID     name      grade 
1      a          80
2      b          95
3      c          85
4      d          96
5      e          98想得到如下的结果:
ID     name      grade 
1      a          
2      b          95
3      c          
4      d          96
5      e          98即是如果grade大于90,grade栏正常显示成绩,如果grade不大于90,则grade显示空,SQL该如何写?

Answer 1

SQL> select * from test;        ID NAME                  GRADE
---------- ---------------- ----------
         5 e                        98
         4 d                        96
         3 c                        85
         2 b                        95
         1 a                        80SQL> 
SQL> select id,name,
  2     case
  3        when grade>=90 then grade
  4        else null
  5     end grade
  6  from test;        ID NAME                  GRADE
---------- ---------------- ----------
         5 e                        98
         4 d                        96
         3 c                
         2 b                        95
         1 a                SQL> 

Answer 2

select id,name,
case when grade > 1400 then grade
     else null
end "grade"
from test;

Answer 3

select ID,name,case when grade>=90 then grade else null end
from test

Answer 4

decode 和 case 都是可以的
顺便学习一下sign函数的用法
sign(x) 
x>0 返回 1
x=0 返回 0
x<0 返回 -1