case when不支援下面2這種寫法(我已經測試過了，大俠如有好辦法，麻煩分享)：

情況1：未加括號

select 
wf.fid id,
CASE 
WHEN wf.Fstep = 1 AND wf.Fstatus = 'Reject' and wf.Foper_reason IS NULL THEN '未填寫駁回理由'
WHEN wf.Fstep = 1 AND wf.Fstatus = 'Reject' and wf.Foper_reason IS NOT NULL THEN wf.Foper_reason
END reason,
wf.Fcreate_time time
from t_work_flow wf 
order by time desc;
 

情況2：加括號

select 
wf.fid id,
CASE 
WHEN (wf.Fstep = 1 AND wf.Fstatus = 'Reject') and wf.Foper_reason IS NULL THEN '未填寫駁回理由'
WHEN (wf.Fstep = 1 AND wf.Fstatus = 'Reject') and wf.Foper_reason IS NOT NULL THEN wf.Foper_reason
END reason,
wf.Fcreate_time time
from t_work_flow wf 
order by time desc;

更改後就可以了

SELECT id, 
CASE when reason is null THEN '未填寫駁回理由' 
ELSE reason 
END reason,
time from 
(
select 
wf.fid id,
CASE 
WHEN wf.Fstep = 1 AND wf.Fstatus = 'Reject' and wf.Foper_reason IS NULL THEN '未填寫駁回理由'
WHEN wf.Fstep = 1 AND wf.Fstatus = 'Reject' and wf.Foper_reason IS NOT NULL THEN wf.Foper_reason
END reason,
wf.Fcreate_time time
from t_work_flow wf
) result
order by time desc;