在单个select语句中，不能使用GROUPBY子句中的select子句设置的别名。

SELECT 
    [user_id],  
    CONVERT(VARCHAR,[start_timestamp], 23) as [start_date] --<< established here
    COUNT([user_id]) AS [user_count]
FROM [dbo].[user_activity]
GROUP BY [user_id],[start_date] --<< illegal reuse of alias here
ORDER BY [start_date]

您可以通过简单地重复别名之前的信息来避免这种情况，如下所示

SELECT 
    [user_id],  
    CONVERT(VARCHAR,[start_timestamp], 23) as [start_date] --<< established here
    COUNT([user_id]) AS [user_count]
FROM [dbo].[user_activity]
GROUP BY [user_id],CONVERT(VARCHAR,[start_timestamp], 23) --<< no alias here
ORDER BY [start_date]

我更希望看到您将时间戳转换为日期，而不是转换为字符串。

SELECT 
    [user_id],  
    cast([start_timestamp] as date) as [start_date] --<< alias established here
    COUNT([user_id]) AS [user_count]
FROM [dbo].[user_activity]
GROUP BY [user_id],cast([start_timestamp] as date)
ORDER BY [start_date]

您需要将转换日期列添加到grı按节向上。

SELECT 
    [user_id],  
    CONVERT(VARCHAR,[start_timestamp], 23) as [start_date]
    COUNT([user_id]) AS [user_count]
  FROM [dbo].[user_activity]
  GROUP BY [user_id],CONVERT(VARCHAR,[start_timestamp], 23)
  ORDER BY [start_date]

列别名 [start_date] 是在 GROUP BY 进行评估。
而是使用派生表（子查询）首先获取 [start_date] ，然后 GROUP BY .

SELECT 
    [user_id],  
    [start_date]
    COUNT([user_id]) AS [user_count]
FROM
(
  SELECT 
      [user_id],
      CONVERT(VARCHAR,[start_timestamp], 23) as [start_date]
  FROM [dbo].[user_activity]
) dt
GROUP BY [user_id], [start_date]
ORDER BY [start_date]

你不应该改成 varchar 没有时间的约会。改为转换为 date .
正如其他人提到的，不能重用 select 中的别名 group by . 你可以重复同样的话 convert 再说一次，但更好的方法是把它放到 cross apply ，这允许您以后重用它。
您还应该使用适当的表别名。

SELECT 
    u.user_id,  
    v.start_date
    COUNT(user_id) AS user_count
  FROM dbo.user_activity u
  CROSS APPLY (VALUES( CAST(u.start_timestamp AS date) ) v(start_date)
  GROUP BY u.user_id, v.start_date
  ORDER BY v.start_date