Question

我有以下查询在Postgres 9.1中很有用：

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;

但我想做的是这样的事情：

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
  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

似乎我不能在order by子句中使用带有CASE语句的聚合latest_interaction的别名。

这有什么变通方法吗？

Answer 1

尝试将其包装为子查询：

SELECT * 
FROM 
(
    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
) 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;

Answer 2

PG手册说ORDER BY表达式：

每个表达式可以是输出列的名称或序号（SELECT列表项），也可以是由输入列值组成的任意表达式。

来自@Mahmoud的子查询解决方案可以使用，或者您可以使用原始列 messages.created_at 或 phone_calls.created_at <创建 ORDER BY /强>

使用别名在ORDER BY中添加Postgres CASE

2 个答案: