因为 workorder 是一张table woactivity 基本上就是那张table上的一个视图 where woclass = 'ACTIVITY' ，您可以通过以下方式简化查询：

select * from worklog where
(class, recordkey, siteid) in (select woclass, wonum, siteid from workorder)
and wonum='M2176'

如果上面的内容在sql server上不适用，请根据@dex的评论，将其翻过来：

select * From worklog where
exists (select 1 from workorder wo where 
    wo.woclass = worklog.class
    and wo.siteid = worklog.siteid
    and wo.wonum = worklog.wonum)
and wonum='M2176'

注意，我添加了 siteid 加入比较组合，因为 workorder 是站点级别的信息。

跳过 case 表达式，使用正则表达式 AND / OR 取而代之的是：

SELECT * from worklog
WHERE (worklog.class = 'ACTIVITY' and
       worklog.recordkey in (select wonum from woactivity where parent='M2176')) 
   OR (worklog.class='WORKORDER' AND 
       worklog.recordkey = (select wonum from workorder where wonum='M2176'))

如上所述：
1） workorder是一个基表，woactivity将根据workorder.woclass属性限制此表的视图
2） workorder对象的逻辑键是siteid+wonum。i、 在整个环境中，wonum值可能不是唯一的。
3） 与特定工单相关的所有记录将具有相同的sited+wogroup值。
4） 主工单记录的istask=0和wonum=wogroup，而关联的活动的istask=1。

你不需要使用 CASE . 我想你可以用布尔运算符来完成你想做的事情

查询1

select * from worklog where 
  (worklog.class ='ACTIVITY' 
    and worklog.recordkey in 
      (select wonum from woactivity where parent ='M2176'))
  OR
  (worklog.class ='WORKORDER'
    and worklog.recordkey in
      (select wonum from workorder where wonum = 'M2176'));

查询2

在这里，我也改变了第二个条件 worklog.recordkey ，来自 = 至 in ```
select * from worklog
where (worklog.class='ACTIVITY'
and worklog.recordkey in
(select wonum from woactivity where parent='M2176'))
or (worklog.class='WORKORDER'
and worklog.recordkey in
(select wonum from workorder where wonum='M2176'))

你也可以用这个。

SELECT * 
FROM worklog w 
WHERE 1=1
    AND w.class = 'ACTIVITY' 
    AND w.recordkey IN (SELECT wonum FROM woactivity WHERE parent = 'M2176')
UNION ALL
SELECT * 
FROM worklog w 
WHERE 1=1
    AND w.class = 'WORKORDER' 
    AND w.recordkey IN (SELECT wonum FROM workorder WHERE parent = 'M2176')