为什么这个 votes/posts 比率总是 1？

Question

我正在与 SEDE 合作创建投票与帖子比率的图表。消除了所有实际错误后，我面临一个新问题：由于某种原因，比率总是 1。这是当前的 SQL:

SELECT CAST(p.CreationDate AS DATE) AS [CreationDate],
       COUNT(CAST(v.CreationDate AS DATE)) / COUNT(CAST(p.CreationDate AS DATE))
         AS [Ratio]
FROM Posts p
INNER JOIN Votes v ON v.PostId = p.Id
WHERE v.VoteTypeId = ##VoteType:int?2## AND
      p.PostTypeId = 1 OR p.PostTypeId = 2
GROUP BY CAST(p.CreationDate AS DATE)
ORDER BY Ratio

可以找到查询本身 here。

suggested in chat 这可能是因为加入表格会产生所有可能的组合，因此投票数和帖子数始终相同（因此 n/n = 1）。这是正确的吗？如果是这样，我应该怎么做？

Answer 1

由于内部联接中的两边都存在，COUNT(CAST(v.CreationDate AS DATE))和COUNT(CAST(p.CreationDate AS DATE))将return完全相同的数字，即组中的行数^*.

如果您想计算在给定日期每个新 post 有多少新选票，请使用 COUNT(DISTINCT):

SELECT CAST(p.CreationDate AS DATE) AS [CreationDate],
       COUNT(DISTINCT v.Id) / COUNT(DISTINCT p.Id) AS [Ratio]
FROM Posts p
INNER JOIN Votes v ON v.PostId = p.Id
WHERE v.VoteTypeId = ##VoteType:int?2## AND
      p.PostTypeId = 1 OR p.PostTypeId = 2
GROUP BY CAST(p.CreationDate AS DATE)
ORDER BY Ratio

^* 假设 CreationDate 不可为空。

为什么这个 votes/posts 比率总是 1？

Why is this votes/posts ratio always 1?

sql

tsql

sql-server

dataexplorer