sql – 使用别名在ORDER BY中添加Postgres CASE

我有以下查询在Postgres 9.1中运行良好：

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？

解决方法

SELECT * 
FROM 
(
    SELECT users.id,GREATEST(
             COALESCE(MAX(messages.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
) Sub
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;

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