选择最新记录

Question

我想运行一个 select 语句，该语句运行并仅通过 Recored_timestampe 字段为键teacher_id 和 student_id 选择最新记录。所以任何时候，它运行它只需要提供一个记录。请问我怎么办？输出可能没有 Recored_timestampe 字段。谢谢

Answer 1

首先按日期时间排列记录

     SELECT *,RANK() OVER (PARTITION BY student_id ORDER BY Recored_timestamp desc) as ranking
     FROM #temp

然后，如果你想知道student_id不为空的最新记录，那么你可以使用OUTER APPLY添加一个非NULL的student_id列。

OUTER APPLY (SELECT student_id 
             FROM #temp 
             WHERE #temp.teacher_id = ranktable.teacher_id 
               AND student_id IS NOT NULL
            ) AS jointable   

这是一个例子：

Create Table #temp
(
     teacher_id int
    ,student_id int
    ,teacher_name varchar(40)
    ,comment varchar(100)
    ,Recored_timestamp datetime
)

INSERT INTO #temp 
VALUES
 (449,111,'lucy','Could be better','2021-05-04 07:00:00.000')
,(449,null,'lucy','smily','2021-05-11 07:00:00.000')
,(449,111,'lucy','not listening','2021-05-08 07:00:00.000')
,(448,null,'Toni','Good','2021-06-04 09:00:00.000')
,(448,222,'Toni','not doing as expected','2021-06-04 08:00:00.000')

SELECT DISTINCT teacher_id,
                jointable.student_id,
                teacher_name,
                comment,
                Recored_timestamp,
                ranking
FROM
(
     SELECT *,RANK() OVER (PARTITION BY teacher_id ORDER BY Recored_timestamp DESC) AS ranking
     FROM #temp
) AS ranktable
OUTER APPLY (SELECT student_id 
             FROM #temp 
             WHERE #temp.teacher_id = ranktable.teacher_id 
               AND student_id IS NOT NULL
            ) AS jointable   
WHERE ranking = 1 --only newest record will be extracted

Drop table #temp

Answer 2

使用窗口函数，按teacher_id和student_id进行分区，然后按recorded_timestamp排序，就会得到想要的结果。

select * from(select teacher_id,student_id,teacher_name,comment ,recorded_timestamp, row_number() over(partition by teacher_id,student_id order by recorded_timestamp desc)as rownum from temp0607)out1 where rownum=1

此外，您可能还需要查看 Recorded_timestamp 的存储方式。如果存储为字符串，则可以使用 from_unixtime(unix_timestamp(recorded_timestamp,'dd/MM/yyyy HH:mm'),'dd/MM/yyyy HH:mm')

将其转换为时间戳

Answer 3

您可以根据此查询获取最新数据。

SELECT TOP 1 * FROM tablename T1 INNER JOIN(SELECT teacher_id, Max(Recored_timestamp) as MaxDate from tablename GROUP BY teacher_id) T2 ON T2。 teacher_id = T1.teacher_id AND T1.Recored_timestamp = T2.MaxDate