Question

我在mysql中有以下两个表：

用户：

+--------+-----------+
| userId | userName  |
+--------+-----------+
| 1      | magnus    |
| 2      | fabiano   |
| 3      | alexander |
| 4      | veselin   |
+--------+-----------+

游戏：

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

我如何构建一个单一的查询，以便我得到以下输出，例如，fabiano的反对者：

输出：

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

Edit1：

这就是我的尝试，而且我无法将它们放入单个查询中：

选择fabiano的对手[从2中的游戏中选择*（userId1，userId2）;]
读取每一行，并检查其中哪一行是fabiano（2），然后选择其他userId
来自这些对手的userIds，从用户表中获取他们的名字

EDIT2：受以下答案的启发，我写了这些（他们的工作）：

-- 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

您可以将两组数据合并在一起，即Fabiano为用户1的所有游戏，以及他所扮演的用户2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 -- Fabiano

    UNION

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

此时假设法比亚诺不能同时成为User1和User2，我们需要考虑UNION ALL vs UNION DISTINCT：）

这也可以整理一下：

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; -- Fabiano

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子句的情况下执行此操作将使其更具性能

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 INNER JOIN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用户标识

   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;

mysql select语句有多个where / conditions

5 个答案: