每组限制分组PostgreSQL

如何解决每组限制分组PostgreSQL

我有以下查询：

WITH relationships AS (
    SELECT related_user_id,count(*) AS trade_count
    FROM trade_history
    WHERE user_id = 487834568 
    GROUP BY related_user_id
    ORDER BY trade_count DESC
)
SELECT offers.*,relationships.trade_count
FROM offers
         LEFT JOIN user_stock
                   ON user_stock.user_id = 487834568 and offers.product_id = user_stock.product_id
         LEFT JOIN relationships
                   ON offers.user_id = relationships.related_user_id
WHERE offers.state = 'OPEN'
  AND offers.user_id != 487834568        
  AND offers.group BETWEEN 1 AND 3 
ORDER BY offers.created_at,relationships.trade_count DESC,user_stock.amount NULLS FIRST;

查询显示我所有报价并通过以下方式订购它们：

• 首先显示较早的优惠
• 首先显示交易量更高的报价（两个用户互相交易）
• 用户库存

我还需要将结果限制为每个product_id最多3个。我进行了一些谷歌搜索，发现使用row_number()和横向联接的窗口函数应该可以实现这一点。我不想使用row_number()，因为表包含很多条目，并且很可能会遇到性能问题。我猜想横向联接是正确的工具（请在此处http://charlesnagy.info/it/postgresql/group-by-limit-per-group-in-postgresql中阅读），但是由于我有限的SQL知识，我无法获得令人满意的结果。

如何以高效的方式进行分组？

编辑： 为了更加清楚，我使用窗口函数实现了该解决方案：

WITH relationships AS (
    SELECT related_user_id,count(*) AS trade_count
    FROM trade_history
    WHERE user_id = 487834568 
    GROUP BY related_user_id
    ORDER BY trade_count DESC
)
SELECT * FROM (
    SELECT 
           offers.*,relationships.trade_count,row_number() OVER (
               PARTITION BY resource_id
               ORDER BY 
                   offers.created_at,user_stock.amount NULLS FIRST
        ) AS row_number
    FROM offers
             LEFT JOIN user_stock
                       ON user_stock.user_id = 487834568 and offers.product_id = user_stock.product_id
             LEFT JOIN relationships
                       ON offers.user_id = relationships.related_user_id
    WHERE offers.state = 'OPEN'
      AND offers.user_id != 487834568        
      AND offers.group BETWEEN 1 AND 3 
                  ORDER BY row_number
              ) AS ordered_offers
WHERE ordered_offers.row_number <= 3;

解决方法

如果您可以在其中添加一些分组，请在HAVING count(offers.product_id) <= 3和WHERE之间添加ORDER BY

将所有内容放在一起，感觉不错。我将尝试基于不同的本地数据库进行类似的分组，并查看其是否按预期工作，否则将进行更新。

WITH relationships AS (
  SELECT related_user_id,count(*) AS trade_count
  FROM trade_history
  WHERE user_id = 487834568 
  GROUP BY related_user_id
  ORDER BY trade_count DESC
)
SELECT
  offers.*,relationships.trade_count
FROM offers
LEFT JOIN user_stock ON
  user_stock.user_id = 487834568 AND
  offers.product_id = user_stock.product_id
LEFT JOIN relationships ON
  offers.user_id = relationships.related_user_id
LEFT JOIN (
  SELECT array_agg(offers.id) AS ids
  FROM offers
  JOIN user_stock ON
    user_stock.user_id = 487834568 AND
    offers.user_id = user_stock.user_id AND
    offers.product_id = user_stock.product_id
  GROUP BY offers.user_id
  HAVING count(offers.id) <= 3
) AS offers_limit ON
  offers.id = any(offers_limit.ids)
WHERE
  offers.state = 'OPEN' AND
  offers.user_id != 487834568 AND
  offers.group BETWEEN 1 AND 3
ORDER BY
  offers.created_at,relationships.trade_count DESC,user_stock.amount NULLS FIRST