mysql查询按顺序和使用连接字符串的并集

我有一个疑问：

SELECT 
  CONCAT(users.id, '#_#', users.nickname, '#_#', users.avatar, '#_#',
         questions.datetime,'#_#',questions.question,'#_#',0000) as summary
FROM
  `questions` join users
  on users.id = questions.id_user 
WHERE
  questions.id = 12 
UNION 
SELECT
  CONCAT(users.id, '#_#', users.nickname, '#_#', users.avatar, '#_#',
         answers.datetime,'#_#',answers.answer,'#_#',answers.id) as summary 
FROM
  answers join users
  on users.id = answers.id_user where id_question = 12

此查询的结果必须是一列（包含串联数据），我需要按答案排序。datetime但我自己找不到解决方案

一些mysql大师能帮我吗

谢谢：）

---

假设您的意思是按

datetime

排序，而不是按

答案排序。顺便问一下，您可以用
问题来简化这些问题。datetime
用于排序？因为
联合的第一部分不加入
应答
。
SELECT * FROM (
  SELECT datetime,
    CONCAT(users.id,'#_#',users.nickname,'#_#',users.avatar,'#_#',questions.datetime,'#_#',questions.question,'#_#',0000) as summary 
  FROM `questions` 
  join users on users.id = questions.id_user 
  WHERE questions.id = 12 
  union 
  select datetime,
    CONCAT(users.id,'#_#',users.nickname,'#_#',users.avatar,'#_#',answers.datetime,'#_#',answers.answer,'#_#',answers.id) as summary 
  from answers 
  join users on users.id = answers.id_user where id_question = 12
) A ORDER BY datetime