INNER JOIN 中的子查询（MySQL Workbench）

Question

我正在开发一个关系数据库，其中包含视频游戏及其用户的信息。我想创建一个视图来显示玩特定游戏（例如 FIFA 22）的用户的名字、姓氏和电子邮件。

我的数据库中有三张表：

1. “NAME”表包含每个游戏的ID（

   id_game

   ），名称（

   name

   ）和描述（

   description

   ）。

2. “PLAY”表包含每个游戏的id（

   id_game

   ）和每个用户的id（

   id_system_user

   ）。

3. “SYSTEM_USER”表包含每个用户的 ID（

   id_system_user

   ）、名字（

   first_name

   ）、姓氏（

   last_name

   ）和电子邮件（

   email

   ）

我已经尝试创建视图，然后在 INNER JOIN 中使用子查询来连接这三个表，但每次执行它时都会出错。代码如下：

CREATE OR REPLACE VIEW vw_FIFA22 AS
    (SELECT S.first_name AS "First name",
            S.last_name AS "Last name",
            S.email AS "E-Mail"
     FROM SYSTEM_USER AS S
     INNER JOIN (SELECT G.name AS "Game name",
                        P.id_system_user AS "System user"
                 FROM GAME AS G
                 INNER JOIN PLAY AS P ON (G.id_game = P.id_game)
                 WHERE G.name = "FIFA 22") AS N
     ON (S.id_system_user = N.id_system_user)
    );

错误是：

Error Code: 1054. Unknown column 'N.id_system_user' in 'on clause'

然后我尝试运行下一段代码并得出：

CREATE OR REPLACE VIEW vw_FIFA22 AS
    (SELECT S.first_name,
            S.last_name,
            S.email
     FROM SYSTEM_USER AS S
     INNER JOIN PLAY AS P
     ON (S.id_system_user = P.id_system_user)
     WHERE id_game = (SELECT id_game
                      FROM GAME
                      WHERE name = "FIFA 22")
    );

此外，在阅读了这个问题的答案后（MySQL INNER JOIN with SubQuery（非常详细）），我尝试运行其他变体并且它也有效：

CREATE OR REPLACE VIEW vw_FIFA22 AS
    (SELECT S.first_name AS "First name",
            S.last_name AS "Last name",
            S.email AS "E-Mail"
     FROM (SELECT P.id_system_user
           FROM GAME AS G
           INNER JOIN PLAY AS P ON (G.id_game = P.id_game)
           WHERE G.name = "FIFA 22") AS P
     INNER JOIN SYSTEM_USER AS S
     ON S.id_system_user = P.id_system_user
    );

为什么这两个变体运行良好但第一个代码却没有？

Answer 1

你只需要将 3 个表连接在一起，不需要子查询例如

CREATE OR REPLACE VIEW vw_FIFA22 AS
(SELECT S.first_name AS "First name",
        S.last_name AS "Last name",
        S.email AS "E-Mail"
 FROM SYSTEM_USER AS S
 INNER JOIN PLAY AS P ON S.id_system_user = P.id_system_user
 INNER JOIN GAME G ON G.id_game = P.id_game
 WHERE G.name = "FIFA 22"
 ;