mySQL查询,用于由连接列限制和排序的离散结果

Question

我有两张桌子

User
  id
  name

Result
  id
  user_id
  score
  created_at

每小时一个cron作业运行并将数据放入每个用户的结果表中 - 根据一些外部变量参数为它们分配一个分数.

因此,Result表包含每个用户的许多记录.

我想在任何给定的点上回顾"前十名"用户.因此,每个用户只应返回一行,并且应该包含最近的可用分数,理想情况下,只有10行与这些分数最高的用户相关.

我目前正在获取所有结果,并使用php进行腿部工作,如果由数据库处理,我相当肯定会更快更高效.但我不知道从哪里开始.

Answer 1

SELECT u.id,
       u.name,
       r.score
FROM Result r
JOIN
  (SELECT user_id, MAX(created_at) AS max_date
   FROM Result
   GROUP BY user_id) lr ON r.user_id=lr.user_id
JOIN User u ON u.id=r.user_id
WHERE r.created_at=lr.max_date
ORDER BY r.score DESC LIMIT 10;

示例运行:

mysql> insert into User (name) values ('foo'), ('bar');
mysql> insert into Result (user_id, score, created_at) values (1,100,'2010-01-20'), (2,150,'2010-01-20'),(1,150,'2010-01-21'),(2,100,'2010-01-21');
mysql> SELECT u.id,        u.name,        r.score FROM Result r JOIN   (SELECT user_id, MAX(created_at) AS max_date    FROM Result    GROUP BY user_id) lr ON r.user_id=lr.user_id JOIN User u ON u.id=r.user_id WHERE r.created_at=lr.max_date ORDER BY r.score DESC LIMIT 10;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | foo  |   150 | 
|  2 | bar  |   100 | 
+----+------+-------+
2 rows in set (0.00 sec)