Question

我有下表名为classamendments，

+ID  |    date    |   time    |   groupnummber    | date added     |  type
============================================================================
 1   |  16-05-18  | 07:00     |   1               | 16/05/ 12:00:00| add 
 2   |  16-05-18  | 07:00     |   1               | 16/05/ 12:05:00| rem
 3   |  16-05-18  | 07:00     |   1               | 16/05/ 12:06:00| add
 4   |  16-05-20  | 15:00     |   4               | 16/05/ 18:49:00| add
 5   |  16-05-20  | 15:00     |   4               | 16/05/ 20:10:00| rem

我如何选择最近的条目，其中日期，时间和组号相同，（第3行和第5行）

Answer 1

一种简单的方法是在where中使用相关子查询：

select t.*
from t
where t.dateadded = (select max(t2.dateadded)
                     from t t2
                     where t2.date = t.date and t2.time = t.time and
                           t2.groupnumber = t.groupnumber
                    );

此查询可以利用t(date, time, groupnumber, dateadded)上的索引，并且使用正确的索引应该非常快。

Answer 2

试试这个：

SELECT t1.*
FROM mytable AS t1
JOIN (
   SELECT date, time, groupnummber, MAX(date_added) AS date_added
   FROM mytable
   GROUP BY date, time, groupnummber 
) AS t2 ON t1.date = t2.date AND
           t1.time = t2.time AND
           t1.groupnummber = t2.groupnummber AND 
           t1.date_added = t2.date_added

选择最近添加的行，其中三列相同

2 个答案: