用于选择多行但每个 PK 的最高值的 SQL 查询

Question

I know that the title sounds horrible but I have no idea how to summarize it better. I'm pretty sure that somebody had the same problem before but I couldn't find anything. RDBMS: MySQL.

Problem: I have the following (simplified) table:

+------+------------+---------------------------------+
| name | date       | score                           |
+------+------------+---------------------------------+
| A    | 01.01.2015 | 1                               |
| A    | 01.02.2015 | 3                               |
| A    | 01.03.2015 | 4                               |
| B    | 01.01.2015 | 3                               |
| B    | 01.02.2015 | 4                               |
| B    | 01.03.2015 | 5                               |
| C    | 01.01.2015 | 1                               |
| C    | 01.02.2015 | 2                               |
| C    | 01.03.2015 | 3                               |
+------+------------+---------------------------------+

There is no unique constraint or PK defined.

The table represents a highscore of a game. Every day the score of all players are inserted with values that are: name, points, now(),...

The data represent a snapshot of the score of each player at a specific time.

I want the most recent entry for each user only but only for the highest X players. So the result should look like

+------+------------+---------------------------------+
| name | date       | score                           |
+------+------------+---------------------------------+
| A    | 01.03.2015 | 4                               |
| B    | 01.03.2015 | 5                               |
+------+------------+---------------------------------+

• C doesn't appear since he's not in the top 2 (by score)
• A appears with the most recent row (by date)
• B appears, like A, with the most recent row (by date) and because he is in the top 2

I hope it becomes clear what I mean.

Thanks in advance!

Answer 1

I understand that what you need is to first select the X players who've gotten the highest score and then get their latest performance. In this case, you should do this:

SELECT * 
FROM tablename t
JOIN
(
  SELECT t.name, max(t.date) as max_date
  FROM tablename t
  JOIN
  (
    SELECT name 
    FROM
    (
      SELECT name, max(score) as max_score
      FROM table_name
      GROUP BY name
    ) all_highscores
    ORDER BY max_score DESC
    LIMIT X
  ) top_scores
  ON top_scores.name = t.name
  GROUP BY t.name
) top_last
on t.name = top_last.name
and t.date = top_last.date;