case when 为空时如何处理

SELECT CASE
  WHEN '' IS NULL THEN 222
  ELSE 0
  END
  FROM DUAL;

这条语句可以正常执行 ，但是下面这条语句就无法正常执行了，请大虾指点

(case receive_detail.USER_DEFINED_03 --列名
  when 'NORMAL' then 'Good'
  when 'BAD' then 'Damaged'
  when '' is null then '/'
  else receive_detail.USER_DEFINED_03
  end) as status,

(case receive_detail.USER_DEFINED_03 --列名
  when 'NORMAL' then 'Good'
  when 'BAD' then 'Damaged'
  when null then '/'
  else receive_detail.USER_DEFINED_03
  end) as status

但是这样达不到想要的效果 ，即空值还是没有被替换成 ‘/’

SQL code

(case when receive_detail.USER_DEFINED_03 ='NORMAL' then 'Good'
when receive_detail.USER_DEFINED_03 ='BAD' then 'Damaged'
when receive_detail.USER_DEFINED_03 is null then '/'
else receive_detail.USER_DEFINED_03
end) as status

这个写法 运行时会报错：ORA-00905: missing keyword

SQL code

--为何不用decode函数呢？
decode(receive_detail.USER_DEFINED_03,'NORMAL','Good',
       'BAD','Damaged',
       '','/',
       receive_detail.USER_DEFINED_03) as status

不会吧，我这个试过了，行的啊
SQL code

with a as(select '1' d from dual union all
select '2'  from dual union all
select '5'  from dual union all
select ''  from dual 
)
select case when d='1' then '11' 
when d='2' then '22' 
when d is null then '/' 
else d end from a

——————————————————————————
11
22
5
/