在 postgresql 中重用子查询

Question

我有一个工作正常的查询，但是我在其中两次使用相同的子查询。我想知道是否有办法重用这个子查询。这是查询：

SELECT DISTINCT homeworks.*
FROM homeworks
INNER JOIN homework_messages ON homeworks.id = homework_messages.homework_id
WHERE homework_messages.message_type = 'submit'
  AND homework_messages.created_at::date <= (CURRENT_DATE - '5 days'::interval)
  AND (
    (
      SELECT MAX(homework_messages.created_at) FROM homework_messages
      WHERE homework_messages.homework_id = homeworks.id AND homework_messages.message_type != 'submit'
    ) < homework_messages.created_at 
    OR 
    (
      SELECT MAX(homework_messages.created_at) FROM homework_messages
      WHERE homework_messages.homework_id = homeworks.id AND homework_messages.message_type != 'submit'
    ) IS NULL
  )
GROUP BY homeworks.id

如您所见，我将这些行重复了两次：

(
  SELECT MAX(homework_messages.created_at) FROM homework_messages
  WHERE homework_messages.homework_id = homeworks.id AND homework_messages.message_type != 'submit'
)

我尝试使用 WITH 子句，但它似乎不适合这个问题，或者我用错了。无论如何，谢谢你的帮助。

Answer 1

这里有一个 cross join lateral 建议。我没有深入研究您的业务逻辑，只是试图使其保持等同。

SELECT DISTINCT homeworks.*
FROM homeworks
INNER JOIN homework_messages ON homeworks.id = homework_messages.homework_id
cross join lateral ( -- your subquery follows
  SELECT MAX(created_at) as created_at FROM homework_messages
  WHERE homework_id = homeworks.id AND message_type != 'submit'
) as lat
WHERE homework_messages.message_type = 'submit'
  AND homework_messages.created_at::date <= (CURRENT_DATE - '5 days'::interval)
  AND (lat.created_at < homework_messages.created_at OR lat.created_at IS NULL)
GROUP BY homeworks.id;

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