来自多个列的 MySQL COUNT 次出现

Question

我有一个名为 games 的 4 人游戏的下表。

+---------+---------+---------+---------+---------+
| game_id | player1 | player2 | player3 | player4 |
+---------+---------+---------+---------+---------+
|    1001 | john    | dave    | NULL    | NULL    |
|    1002 | dave    | john    | mike    | tim     |
|    1003 | mike    | john    | dave    | NULL    |
|    1004 | tim     | dave    | NULL    | NULL    |
+---------+---------+---------+---------+---------+

我想回答两个问题:

1. 谁参加的比赛最多？ (戴夫)
2. 哪对球员一起打的比赛最多？ (约翰和戴夫)

对于 #1，我尝试调整我在这里找到的答案:mySQL query to find the most repeated value但它似乎只能回答一个专栏的问题。这意味着我可以了解谁是 player1 最多的人，但不是任何玩家中玩游戏次数最多的人:

SELECT player1 p1, COUNT(*) p1 FROM games
GROUP BY p1    
ORDER BY p1 DESC;

有没有办法将这些列连接在一起，或者我必须在应用程序代码中处理吗？

不确定#2 从哪里开始。我想知道我的表结构是否应该将玩家合并到一个列中:

+----+---------+--------+
| id | game_id | player |
+----+---------+--------+
|  1 |    1001 | john   |
|  2 |    1001 | dave   |
|  3 |    1002 | john   |
|  4 |    1002 | dave   |
|  5 |    1002 | mike   |
|  6 |    1002 | tim    |
+----+---------+--------+

Answer 1

最好的办法是规范化数据库。这是一个多对多的关系，需要一个链接表来将一个游戏连接到它对应的玩家。那么计算就会容易得多。不过，您可以为第一个问题使用派生表，将所有列合并为一个:

SELECT `player`,
       COUNT(*) as `count`
FROM
    (
        SELECT `player1` `player`
        FROM `games`
        UNION ALL
        SELECT `player2` `player`
        FROM `games`
        UNION ALL
        SELECT `player3` `player`
        FROM `games`
        UNION ALL
        SELECT `player4` `player`
        FROM `games`
    ) p
GROUP BY `player` HAVING `player` IS NOT NULL
ORDER BY `count` DESC

参见 live demo here

对于第二个问题，您必须在派生表上进行内部联接:

SELECT `p`.`player`,
       `p2`.`player`,
       count(*) AS count
FROM
    (
        SELECT `game_id`, `player1` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player2` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player3` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player4` `player`
        FROM `games`
    ) p
INNER JOIN
    (
        SELECT `game_id`, `player1` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player2` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player3` `player`
        FROM `games`
        UNION ALL
        SELECT `game_id`, `player4` `player`
        FROM `games`
    ) p2
ON `p`.`game_id` = `p2`.`game_id` AND `p`.`player` < `p2`.`player`
WHERE `p`.`player` IS NOT NULL AND `p2`.`player` IS NOT NULL
GROUP BY `p`.`player`, `p2`.`player`
ORDER BY `count` DESC

参见 live demo here