[英]How to select only the latest rows for each user?
My table looks like this: 我的表看起来像这样:
id | user_id | period_id | completed_on
----------------------------------------
1 | 1 | 1 | 2010-01-01
2 | 2 | 1 | 2010-01-10
3 | 3 | 1 | 2010-01-13
4 | 1 | 2 | 2011-01-01
5 | 2 | 2 | 2011-01-03
6 | 2 | 3 | 2012-01-13
... | ... | ... | ...
I want to select only the latest users periods entries, bearing in mind that users will not all have the same period entries. 我想只选择最新的用户期间条目,记住用户不会都有相同的期间条目。
Essentially (assuming all I have is the above table) I want to get this: 基本上(假设我只有上表)我想得到这个:
id | user_id | period_id | completed_on
----------------------------------------
3 | 3 | 1 | 2010-01-13
4 | 1 | 2 | 2011-01-01
6 | 2 | 3 | 2012-01-13
Both of the below queries always resulted with the first user_id occurance being selected, not the latest (because the ordering happens after the rows are selected from what I understand): 以下两个查询总是在第一个user_id出现被选中时产生,而不是最新的(因为在从我理解的行中选择行之后排序发生):
SELECT
DISTINCT user_id,
period_id,
completed_on
FROM my_table
ORDER BY
user_id ASC,
period_id DESC
SELECT *
FROM my_table
GROUP BY user_id
ORDER BY
user_id ASC,
period_id DESC
Seems like this should work using MAX
and a subquery: 看起来这应该使用MAX
和子查询:
SELECT t.Id, t.User_Id, t.Period_Id, t.Completed_On
FROM my_table t
JOIN (SELECT Max(completed_on) Max_Completed_On, t.User_Id
FROM my_table
GROUP BY t.User_ID
) t2 ON
t.User_Id = t2.User_Id AND t.Completed_On = t2.Max_Completed_On
However, if you potentially have multiple records where the completed_on date is the same per user, then this could return multiple records. 但是,如果您可能有多个记录,其中每个用户的completed_on日期相同,则可能会返回多个记录。 Depending on your needs, potentially adding a MAX(Id) in your subquery and joining on that would work. 根据您的需要,可能会在子查询中添加MAX(Id)并加入其中。
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