Question

ORDER BY子句不使用Union，别名（as）得到一些错误

Messenger表

       mid    senderid   receiverid   message   
      ---------------------------------------
        4       100       200          hi
        3       200       100          hello
        2       100       200          hi
        1       100       200          hi

选择语句

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

错误

   #1054 - Unknown column 'mid' in 'order clause'

我不知道我犯了什么错误，请提前谢谢你

Answer 1

在你的联合结果中，你没有中间栏你只有new_id 你应该添加

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

或仅使用单个查询避免联合

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

Answer 2

您为什么使用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;

Answer 3

您的联合查询中没有中间字段，因此要么将其设为

 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

 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

SQL错误ORDER BY子句不起作用

3 个答案: