SELECT users.id,GREATEST(
 COALESCE(MAX(messages.created_at),'2012-07-25 16:05:41.870117'),COALESCE(MAX(phone_calls.created_at),'2012-07-25 16:05:41.870117')
) AS latest_interaction
FROM users LEFT JOIN messages ON users.id = messages.user_id
LEFT JOIN phone_calls ON users.id = phone_calls.user_id
GROUP BY users.id
ORDER BY latest_interaction DESC
LIMIT 5;

但我想要做的是这样的事情：

SELECT users.id,'2012-07-25 16:05:41.870117')
) AS latest_interaction
FROM users LEFT JOIN messages ON users.id = messages.user_id
LEFT JOIN phone_calls ON users.id = phone_calls.user_id
GROUP BY users.id
ORDER BY
  CASE WHEN(
    latest_interaction > '2012-09-05 16:05:41.870117')
  THEN 0
  WHEN(latest_interaction > '2012-09-04 16:05:41.870117')
  THEN 2
  WHEN(latest_interaction > '2012-09-04 16:05:41.870117')
  THEN 3
  ELSE 4
  END
LIMIT 5;

我收到以下错误：
错误：列“latest_interaction”不存在

似乎我不能使用带有CASE语句的order by子句中的聚合latest_interaction的别名.

这有什么变通方法吗？