MySQL 查询对

Question

如何选择所有尚未与 'alpha' 配对的玩家？ （输出='伽玛'）。即使对表为空，它也应该工作。

table: players
+----------+-------+
| playerID | name  |
+----------+-------+
|        1 | alpha |
|        2 | beta  |
|        3 | gamma |
+----------+-------+

table: pairs
+---------+---------+
| player1 | player2 |
+---------+---------+
|       2 |       3 |
|       1 |       2 |
+---------+---------+

我已经挣扎了好几个小时了。例如如果我这样做

SELECT p.*, r.*
FROM players p
JOIN pairs r
    ON (player1 = playerID) OR (player2 = playerID)
WHERE
    ((r.player1 != 1) AND (r.player2 != 1));

输出是“beta”和“gamma”。在连接中，“beta”出现两次（每次与“alpha”和“gamma”配对）。并且 WHERE 条件消除了一行“beta”。我想要的是消除所有“beta”行。我是新手，尝试了 GROUP BY、HAVING 等的各种组合。我无法让它工作。

SQL Fiddle here.

Answer 1

我会把它分解成更小的部分，然后拼凑起来。首先获取 alpha 玩家的 id：

SELECT playerID
FROM players
WHERE name = 'alpha';

下一步是弄清楚如何排除。我将获得与玩家 1 配对的所有玩家的列表。我通过选择 player1 为“alpha”的所有 player2 值和 player2 为“alpha”的所有 player1 值来做到这一点：

SELECT p.player2
FROM pairs p
JOIN(
  SELECT playerID
  FROM players
  WHERE name = 'alpha') b ON b.playerID = p.player1
UNION
SELECT p.player1
FROM pairs p
JOIN(
  SELECT playerID
  FROM players
  WHERE name = 'alpha') b ON b.playerID = p.player2;

完成此操作后，剩下的唯一步骤就是从该人 1) 不是 alpha 和 2) 不在上述列表中的玩家中拉出：

SELECT *
FROM players
WHERE playerID NOT IN(
  SELECT playerID
  FROM players
  WHERE name = 'alpha') 
AND playerID NOT IN(
  SELECT p.player2
  FROM pairs p
  JOIN(
    SELECT playerID
    FROM players
    WHERE name = 'alpha') b ON b.playerID = p.player1
  UNION
  SELECT p.player1
  FROM pairs p
  JOIN(
    SELECT playerID
    FROM players
    WHERE name = 'alpha') b ON b.playerID = p.player2);

SQL Fiddle 此处为示例。

Answer 2

这样的事情应该适用于这种情况。

SELECT
    pl.*
FROM 
    players as pl
WHERE 
    pl.playerID NOT IN 
    (
      SELECT 
          p.player2
      FROM
          pairs AS p
      INNER JOIN 
          players plr ON plr.playerID = p.player1
      WHERE 
          plr.name='alpha'

    )

如果对表中的 player1 和 player 2 列改变位置，您可能必须在子查询中使用 CASE。

Answer 3

应该这样做：

drop table pair;
drop table player;

create table player (
    id int,
    name varchar(32)
);

create table pair (
    id1 int,
    id2 int
);

insert into player values (1, 'Andy');
insert into player values (2, 'Bob');
insert into player values (3, 'Carl');
insert into player values (4, 'Dave');

insert into pair values (2, 3);
insert into pair values (1, 2);
insert into pair values (3, 1);

select * from player where id not in (
select
    if(pair.id1 = player.id, pair.id2, pair.id1) as other_player
from 
    pair
    join player on pair.id1 = player.id or pair.id2 = player.id
where
    player.name = 'Andy'
) 
;

Answer 4

另一种解决方案，不使用“NOT IN”。

方法：（所有用户）-（所有现有对手）

即使（所有现有对手）为空，这也有效。

SELECT p.* FROM players p
LEFT JOIN (
    SELECT CASE
            WHEN (player1 = 1) THEN player2
            WHEN (player2 = 1) THEN player1
        END AS opponentID
    FROM pairs) existingOpponents
ON (p.playerID = existingOpponents.opponentID)
WHERE (existingOpponents.opponentID IS NULL) AND 
    (p.playerID != 1);