具有多个 where/条件的 mysql select 语句

Question

我在mysql中有以下两个表：

用户：

+--------+-----------+
| userId | userName  |
+--------+-----------+
| 1      | magnus    |
| 2      | f*o   |
| 3      | alexander |
| 4      | veselin   |
+--------+-----------+

游戏：

+--------+---------+---------+
| gameId | userId1 | userId2 |
+--------+---------+---------+
| 1      | 1       | 2       |
| 2      | 1       | 3       |
| 3      | 2       | 3       |
| 4      | 2       | 4       |
+--------+---------+---------+

我怎样才能构造一个单一的查询，这样我才能得到下面的输出，比如说，f*o 的对手：

输出：

+--------+-----------+
| gameId | userName  |
+--------+-----------+
| 1      | magnus    |
| 3      | alexander |
| 4      | veselin   |
+--------+-----------+

编辑1：

这是我正在尝试的，但我无法将它们放入单个查询中：

• 选择 f*o 的对手 [select * from games where 2 in (userId1, userId2);]
• 读取每一行，检查哪一行是f*o(2)，然后选择另一个userId
• 从这些对手的 userIds 中，从 users 表中获取他们的名字

编辑2： 受以下答案的启发，我写了这个（它们有效）：

-- NO JOIN
select x.gameId, users.userName from
(
select gameId, userId2 as id from games where userId1=2
UNION
select gameId, userId1 as id from games where userId2=2 
) as x, users 
where users.userId = id;

-- NO JOIN, NO UNION        
select x.gameId, users.userName from (
SELECT g.gameId,
    CASE WHEN userId1 = 2
            THEN userId2     
         WHEN userId2 =2
            THEN userId1
         END AS id
FROM games g) as x, users
where users.userId = id;

Answer 1

您可以将这两组数据合并在一起，即 F*o 是用户 1 的所有游戏，以及他扮演用户 2 角色的所有游戏：

SELECT x.Opponent
FROM
(
    SELECT u.Name AS Opponent
    FROM games g
    INNER JOIN users u
    ON g.userId2 = u.UserId
    WHERE g.UserId1 = 2 -- F*o

    UNION

    SELECT u.Name
    FROM games g
    INNER JOIN users u
    ON g.userId1 = u.UserId
    WHERE g.UserId2 = 2 -- F*o
) AS x;

此时假设 F*o 不能同时是 User1 和 User2，因为我们需要考虑 UNION ALL vs UNION DISTINCT :)

这也可以整理成：

SELECT x.Opponent
FROM
(
    SELECT u.Name AS Opponent, g.UserId1 AS PlayerId
    FROM games g
    INNER JOIN users u
    ON g.userId2 = u.UserId

    UNION

    SELECT u.Name, g.UserId2 AS PlayerId
    FROM games g
    INNER JOIN users u
    ON g.userId1 = u.UserId
) AS x
WHERE x.PlayerId = 2; -- F*o

Answer 2

尝试类似：

SELECT `gamess`.gameId, `users`.userName
FROM users INNER JOIN
   (SELECT gameId, userId2 as userId
    FROM games
    WHERE userId1 = 2
    UNION
    SELECT gameId, userId1 as userId
    FROM games
    WHERE userId2 = 2) AS gamess
ON `gamess`.userId = `users`.userId

Answer 3

在没有 UNION 子句的情况下这样做会提高性能

SELECT g.gameid,
CASE WHEN u1.userid = 2 -- fabino* 
            THEN u2.username
            else u1.username END AS Opponent
FROM games g
LEFT JOIN users u1
ON g.userId1 = u1.UserId
LEFT JOIN users u2
on g.userid2 = u2.userid
WHERE (g.UserId1 = 2 OR g.userid2 = 2) -- fabino

Answer 4

选择我们。* 来自用户我们 内部加入游戏 gs ON us.userId = gs.userId1 GROUP BY us.userId ,us.userName

Answer 5

此查询应该适用于所有常见的用户 ID

   SELECT x.Opponent
  FROM
  ( 
    SELECT u.userName AS Opponent
    FROM games g
    INNER JOIN users u
    ON g.userId2 = u.UserId
    WHERE g.UserId1 in (select UserId from users where UserId in (select userid1 from games)) 
    UNION
    SELECT u.userName
    FROM games g
    INNER JOIN users u
    ON g.userId1 = u.UserId
    WHERE g.UserId2 in (select UserId from users where UserId in (select userid2 from games))  
  ) AS x;