限制查询结果

Question

我有一个包含音乐排行榜的应用程序来展示顶级曲目（它显示前10名）。

但是，我试图限制图表，以便任何特定的用户不能同时在顶部图表上有多个轨道。

如果您需要更多信息，请让我知道。

Answer 1

您可以使用row_number()函数，该函数提供一个在用户标识更改时重置的运行编号。然后你可以使用在WHERE子句创建每个用户的限制：

SELECT * FROM (
    SELECT COALESCE(sum(plays.clicks), 0), 
     row_number() OVER (PARTITION BY users.id ORDER BY COALESCE(sum(plays.clicks), 0) DESC), 
     users.id AS user_id, 
     tracks.* 
     FROM tracks 
      JOIN plays 
      ON tracks.id = plays.track_id 
      AND plays.created_at > now() - interval '14 days' 
      INNER JOIN albums 
      ON tracks.album_id = albums.id 
      INNER JOIN users 
      ON albums.user_id = users.id 
      GROUP BY users.id, tracks.id 
      ORDER BY 1 desc) sq1 
    WHERE row_number <= 2 LIMIT 10;