为什么我得到列“结束”不存在？

Question

我正在尝试在 SQL 中执行条件 ORDER BY，但出现错误：column "ended" does not exist 指向查询的“结束”部分的情况。

这是我的查询：

SELECT (EXTRACT(EPOCH FROM SUBMISSION_DEADLINE)
       -EXTRACT(EPOCH FROM now())) AS "datediff"
    , (EXTRACT(EPOCH FROM SUBMISSION_DEADLINE)>EXTRACT(EPOCH FROM now())) AS "ended", *
FROM imdown_api_v1_contest
ORDER BY "ended" DESC, case when "ended" = true then "datediff" else "-datediff" END ASC;

我需要先抓住尚未结束的比赛，然后按最先结束的比赛排序。然后我需要所有已经结束的，并按最近结束的排序。

Answer 1

执行order by时未定义列ended，sql按下一个顺序执行查询：

1. 来自
2. 在哪里
3. 订购
4. 选择

因此，您需要将完整表达式复制到 order by 子句中或使用子查询，例如：

select * from (
    SELECT (EXTRACT(EPOCH FROM SUBMISSION_DEADLINE)-EXTRACT(EPOCH FROM now())) AS "datediff", (EXTRACT(EPOCH FROM SUBMISSION_DEADLINE)>EXTRACT(EPOCH FROM now())) AS "ended", *
    FROM imdown_api_v1_contest
) tmpq
ORDER BY "ended" DESC, case when "ended" = true then "datediff" else "-datediff" END ASC

Answer 2

您的查询在多个地方被破坏或效率低下，而不仅仅是对列别名的非法引用。这可能会如你所愿：

SELECT EXTRACT(EPOCH FROM now() - submission_deadline) AS datediff
     , now() > submission_deadline AS ended  -- I inverted your expression!
     , *
FROM   imdown_api_v1_contest
ORDER  BY 2  -- positional reference to output column
     , @(EXTRACT(EPOCH FROM now() - submission_deadline))

通常，“结束”的比赛在过去中有一个submission_deadline，所以now() > submission_deadline AS ended，而不是相反。

为了修复您的主要语法错误，我使用位置引用而不是列别名。详细解释：

• PostgreSQL Where count condition

相关示例：

• PostgreSQL: running count of rows for a query 'by minute'

为了达到您想要的订单，我使用absolute value operator @。