SQL：如何进行查询以返回每个用户从表数据中最后创建的行

Question

Consider following table's data

ID      UserID  ClassID SchoolID    Created
2184    19313   10      28189       2010-10-25 14:16:39.823
46697   19313   10      27721       2011-04-04 14:50:49.433
•47423  19313   11      27721       2011-09-15 09:15:51.740
•47672  19881   11      42978       2011-09-19 17:31:12.853
3176    19881   11      42978       2010-10-27 22:29:41.130
22327   19881   9       45263       2011-02-14 19:42:41.320
46661   32810   11      41861       2011-04-04 14:26:14.800
•47333  32810   11      51721       2011-09-13 22:43:06.053
131     32810   11      51721       2010-09-22 03:16:44.520

I want to make a sql query that return the last created row for each UserID in which the result will be as below ( row that begin with • in the above rows ) :

ID      UserID  ClassID SchoolID    Created
47423   19313   11      27721       2011-09-15 09:15:51.740
47672   19881   11      42978       2011-09-19 17:31:12.853
47333   32810   11      51721       2011-09-13 22:43:06.053

Answer 1

You can use a CTE (Common Table Expression) with the ROW_NUMBER function:

;WITH LastPerUser AS
(
   SELECT 
       ID, UserID, ClassID, SchoolID, Created,
       ROW_NUMBER() OVER(PARTITION BY UserID ORDER BY Created DESC) AS 'RowNum')
   FROM dbo.YourTable
)
SELECT 
   ID, UserID, ClassID, SchoolID, Created,
FROM LastPerUser
WHERE RowNum = 1

This CTE "partitions" your data by UserID , and for each partition, the ROW_NUMBER function hands out sequential numbers, starting at 1 and ordered by Created DESC - so the latest row gets RowNum = 1 (for each UserID ) which is what I select from the CTE in the SELECT statement after it.

Answer 2

I know this is an old question at this point, but I was having the same problem in MySQL, and I think I have figured out a standard sql way of doing this. I have only tested this with MySQL, but I don't believe I am using anything MySQL-specific.

select mainTable.* from YourTable mainTable, (
    select UserID, max(Created) as Created
    from YourTable
    group by UserID
) dateTable 
where mainTable.UserID = dateTable.UserID
and mainTable.Created = dateTable.Created