问题描述

我在留言簿中有一张包含用户评论的表格.列是:id、user_id、title、comment、timestamp.

I have a table with user comments in a guestbook. Columns are: id, user_id, title, comment, timestamp.

我需要为每个用户选择最新的行.我曾尝试使用 group by 执行此操作，但尚未对其进行管理，因为我无法在按 user_id 分组的同一查询中选择任何其他内容:

I need to select the latest row for each user. I have tried to do this with group by but havent managed it because i cant select anything else in the same query where i group by user_id:

SELECT user_id, MAX(ts) FROM comments GROUP BY user_id

例如在这个查询中，我不能添加到选择列 id、tilte 和 comment.这怎么办?

for example in this query i cant add to also select columns id, tilte and comment. How can this be done?

推荐答案

可以使用解析函数

SELECT *
  FROM (SELECT c.*,
               rank() over (partition by user_id order by ts desc) rnk
          FROM comments c)
 WHERE rnk = 1

根据您希望如何处理关系(如果可以有两行具有相同的 user_id 和 ts)，您可能需要使用 row_number 或 dense_rank 函数而不是 rank.如果有平局，rank 将允许多行排在第一位.如果有平局，row_number 将任意返回一行.对于并列第一的行，dense_rank 的行为类似于 rank，但会认为下一行是第二行，而不是第三行，假设两行并列第一.

Depending on how you want to handle ties (if there can be two rows with the same user_id and ts), you may want to use the row_number or dense_rank function rather than rank. rank would allow multiple rows to be first if there was a tie. row_number would arbitrarily return one row if there was a tie. dense_rank would behave like rank for the rows that tied for first but would consider the next row to be second rather than third assuming two rows tie for first.

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