Question

给出以下架构：

CREATE TABLE players (
  id BIGINT PRIMARY KEY,
  name TEXT UNIQUE
);

CREATE TABLE trials (
  timestamp TIMESTAMP PRIMARY KEY,
  player BIGINT,
  score NUMERIC
);

如何创建SELECT，首先从trials找到最佳分数，然后加入name的{{1}}字段？在使用此查询后，我已经能够获得分数：

users

我对返回前10个分数的查询如下：

SELECT * FROM trials GROUP BY player ORDER BY score ASC LIMIT 10;

但是当我在表中搜索数千个条目时，数据库性能开始爬行。我认为子查询正在扼杀我的表现。第一个查询（仅返回最高分数）仍然可以充分执行，因此我想知道是否有办法在选择最高分后强制CREATE VIEW top10place AS SELECT player.name AS name, trial.* FROM trials AS trial, players AS player WHERE trial.player = player.id AND trial.score = ( SELECT MAX(score) FROM trials AS tsub WHERE tsub.player = trial.player ) ORDER BY trial.score DESC, trial.timestamp ASC LIMIT 10;操作。

编辑请注意，该查询将返回排名前10位的玩家，而不仅仅是排名前10位的玩家。如果同一个玩家有很多高分，他应该只出现在前10名中。

我正在使用SQLite，因此它没有SQL Server或MySQL的某些扩展功能。

Answer 1

没有运行sqlite，希望限制是正确的。

select players.name, trials.player, trials.timestamp, trials.score from
    (select player, score, timestamp from
    trials order by score desc, timestamp asc limit 10) trials, players
where players.id = trials.player

此致

Answer 2

这是你做出比它需要更难的事情的一个例子。正确的代码是：

CREATE VIEW top10place AS
  SELECT player.name AS name, trial.*
    FROM trials AS trial, players AS player
    WHERE trial.player = player.id
    ORDER BY trial.score ASC, trial.timestamp ASC
    LIMIT 10;

基本上，让LIMIT语句完成工作：）

Answer 3

如果优化器为每一行运行它，那么WHERE中的子查询可能会很昂贵。

（编辑）这是编写查询的另一种方法，现在使用独占连接：它表示该用户没有排得分更高的行：

SELECT 
     (select name from user where id = cur.userid) as UserName
,    cur.score as MaxScore
FROM trails cur
LEFT JOIN trials higher
    ON higher.userid = cur.userid
    AND higher.timestamp <> cur.timestamp
    AND higher.score > cur.score
WHERE higher.userid is null
ORDER BY cur.score DESC
LIMIT 10

这将返回10个得分最高的用户。如果您不管用户是否想要10个最高分，请查看Silas的答案。

Answer 4

正如已经提到的，由于你在玩家和试验之间的识别关键是player.id和trials.player，你应该在trial.player上有一个索引。特别是如果你把这两张桌子关联起来的话。

您也可以尝试使查询更像。

SELECT p.name as name, t.* FROM players as p
INNER JOIN (SELECT * FROM trials WHERE trials.score = (SELECT MAX(score) FROM trials as tsub WHERE tsub.player = trials.player) LIMIT 10) as t ON t.player = p.id
ORDER BY t.score DESC, t.timestamp ASC

这甚至可以进行一些优化，但如果没有一些数据可以将查询抛出，我就不擅长了。

处理SELECT后加入

4 个答案: