联合查询中没有中间字段，所以

select senderid as new_id from messenger  where  receiverid=200
  union 
  select  receiverid as new_id from messenger  where  senderid =200
  order by new_id  desc

或者

select mid,senderid as new_id from messenger  where  receiverid=200
  union 
  select  mid,receiverid as new_id from messenger  where  senderid =200
  order by mid  desc

在你的联队成绩中你没有中锋
你只有 new_id 你应该加上

select mid, senderid as new_id 
from messenger  where  receiverid=200
union 
select mid, receiverid 
from messenger  where  senderid =200
order by mid  desc

或者避免仅仅使用单个查询进行联合

select case when senderid = 200 then reciverid else senderid as new_id 
from messenger  
where  receiverid=200 or senderid = 200
order by mid

你为什么用这个 union ?

select (case when receiverid = 200 then senderid else receiverid end) as new_id
from messenger m
where 200 in (receiverid, senderid)
group by new_id
order by min(mid) desc;