假设我有一个可以由 2、3 或 4 名玩家玩的游戏。我在我的数据库 (MySQL 5.1) 中的三个表中跟踪此类游戏，如下所示。我希望这些字段是不言自明的:

create table users (id int, login char(8));
create table games (id int, stime datetime, etime datetime);
create table users_games (uid int, gid int, score int);

[游戏表中跟踪的两个时间是开始时间和结束时间]

这里是一些填充表格的虚拟数据:

insert into games values
(1, '2011-12-01 10:00:00', '2011-12-01 13:00:00'),
(2, '2011-12-02 11:00:00', '2011-12-01 14:00:00'),
(3, '2011-12-03 12:00:00', '2011-12-01 15:00:00'),
(4, '2011-12-04 13:00:00', '2011-12-01 16:00:00');

insert into users_games values
(101, 1, 10),
(102, 1, 11),
(101, 2, 12),
(103, 2, 13),
(104, 2, 14),
(102, 3, 15),
(103, 3, 16),
(104, 3, 17),
(105, 3, 18),
(102, 4, 19),
(104, 4, 20),
(105, 4, 21);

现在，我需要生成以下格式的报告:

gid     p1    p2    p3    p4  started ended
1      101   102               [g1]    [g1]
2      101   103   104         [g2]    [g2]
3      102   103   104   105   [g3]    [g3]
4      102   104   105         [g4]    [g4]

也就是显示同一行所有玩过游戏的玩家的报告。我还需要他们的分数和用户表中的一些其他信息，但这是第 2 阶段。:-)

我从这个开始:

select g.id, g.stime, g.etime, ug1.uid, ug2.uid, ug3.uid, ug4.uid
from games g, users_games ug1, users_games ug2, users_games ug3, users_games ug4
where
g.id = ug1.gid and
ug1.gid = ug2.gid and
ug1.uid < ug2.uid and
ug2.gid = ug3.gid and
ug2.uid < ug3.uid and
ug3.gid = ug4.gid and
ug3.uid < ug4.uid

这给了我所有四个座位都被占用的游戏(即，上述虚拟数据中只有游戏 ID 3)。但这只是我需要的数据的一部分。

这是我的第二次尝试:

select g.id, g.stime, g.etime, ug1.uid, ug2.uid,
    ifnull(ug3.uid, ''), ifnull(ug4.uid, '')
from ( games g, users_games ug1, users_games ug2 )
left join users_games ug3 on ug2.gid = ug3.gid and ug2.uid < ug3.uid
left join users_games ug4 on ug3.gid = ug4.gid and ug3.uid < ug4.uid
where
g.id = ug1.gid and
ug1.gid = ug2.gid and
ug1.uid < ug2.uid

这给了我 14 行上面的虚拟数据。我试图通过将 ug1 锚定到最低 UID 播放器的条目来消除一个错误来源:

select g.id, g.stime, g.etime, ug1.uid, ug2.uid,
    ifnull(ug3.uid, ''), ifnull(ug4.uid, '')
from
( games g, users_games ug1, users_games ug2,
    (select gid as g, min(uid) as u from users_games group by g) as xx
)
left join users_games ug3 on ug2.gid = ug3.gid and ug2.uid < ug3.uid
left join users_games ug4 on ug3.gid = ug4.gid and ug3.uid < ug4.uid
where
g.id = xx.g and
ug1.uid = xx.u and
g.id = ug1.gid and
ug1.gid = ug2.gid and
ug1.uid < ug2.uid

现在我减少到 9 行，但我仍然有很多虚假数据。我可以看到问题 - 例如在游戏 3 中，当 ug1 锚定到用户 102 时，仍然有三个玩家可以锚定 ug2。等等。但是我想不出解决这个难题的方法 - 我怎样才能最终实现一个查询，以正确的顺序和数字输出 4 行玩家？

在我看来，这在其他情况下应该是一个已解决的问题。在此感谢所有帮助。

最佳答案

您遇到的一个问题是您没有将用户描述为玩家 1、2、3 或 4 的字段。但是，您需要确保每个 LEFT JOIN 只加入一个玩家。

如果您向 users_games 添加一个“player_id”字段，它就变得微不足道了...

SELECT
  *
FROM
  games
LEFT JOIN
  users_games      AS p1
    ON  p1.gid = games.id
    AND p1.player_id = 1
LEFT JOIN
  users_games      AS p2
    ON  p2.gid = games.id
    AND p2.player_id = 2
LEFT JOIN
  users_games      AS p3
    ON  p3.gid = games.id
    AND p3.player_id = 3
LEFT JOIN
  users_games      AS p4
    ON  p4.gid = games.id
    AND p4.player_id = 4

有替代方案可以避免所有 LEFT JOIN，但这个例子很好用，因为它是下一步的基础...)

如果你不能添加这个字段，它会变得更复杂。 (SQL Server、Oracle 等可以使用 ROW_NUMBER() 代理此 player_id 字段，MySQL 不能。)

相反，您需要相关的子查询来识别“下一个玩家”。

SELECT
  *
FROM
  games
LEFT JOIN
  users_games      AS p1
    ON  p1.gid = games.id
    AND p1.uid = (SELECT MIN(uid) FROM users_games WHERE gid = games.id)
LEFT JOIN
  users_games      AS p2
    ON  p2.gid = games.id
    AND p2.uid = (SELECT MIN(uid) FROM users_games WHERE gid = games.id AND uid > p1.uid)
LEFT JOIN
  users_games      AS p3
    ON  p3.gid = games.id
    AND p3.uid = (SELECT MIN(uid) FROM users_games WHERE gid = games.id AND uid > p2.uid)
LEFT JOIN
  users_games      AS p4
    ON  p4.gid = games.id
    AND p4.uid = (SELECT MIN(uid) FROM users_games WHERE gid = games.id AND uid > p3.uid)

编辑 加入免费版本，假设存在 player_id 字段...

SELECT
  games.id,
  MAX(CASE WHEN users_games.player_id = 1 THEN users_games.uid END)   AS p1_id,
  MAX(CASE WHEN users_games.player_id = 2 THEN users_games.uid END)   AS p2_id,
  MAX(CASE WHEN users_games.player_id = 3 THEN users_games.uid END)   AS p3_id,
  MAX(CASE WHEN users_games.player_id = 4 THEN users_games.uid END)   AS p4_id
FROM
  games
LEFT JOIN
  users_games
    ON users_games.gid = games.id
GROUP BY
  games.id

关于mysql - 多次左连接同一个表，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/8564486/