mysqlcasewhen多条件同时满⾜的多个and组合嵌套的情况，
isnull的用法判断空isnull---系列⼀
⼆
case when不⽀持下⾯2这种写法(我已经测试过了，⼤侠如有好办法，⿇烦分享)：
情况1：未加括号
select
wf.fid id,
CASE
WHEN wf.Fstep = 1 AND wf.Fstatus = 'Reject' and wf.Foper_reason IS NULL THEN '未填写驳回理由'
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;
情况2：加括号
select
wf.fid id,
CASE
WHEN (wf.Fstep = 1 AND wf.Fstatus = 'Reject') and wf.Foper_reason IS NULL THEN '未填写驳回理由'
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;
更改后就可以了
SELECT id,
CASE when reason is null THEN '未填写驳回理由'
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 '未填写驳回理由'
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;

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