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mysql - 连接同一个表中的两个外键引用同一个主键

问题描述

我正在尝试将多个表格BooksReview（Followers_ Books_ _ _ _ _ _想想问题出在哪里）有两个来自同一个主键的引用，它来自表中，也来自表中。UsersBooksuserIdUsersBooksReviewuserIduserIdbookIdBooksFollowersfolloweruserIdUsersfolloweduserIdUsers

问题：我创建了一个 MySQL 查询来获取基于特定用户的特定书籍的书评数量，并获取该用户的书籍数量和他/她拥有的关注者数量，但是当我将Followers连接部分添加到我的查询显示所有值的结果为 0，预期值为 4 本书、4 条评论和 1 个关注者。

我试图更改查询中的连接类型，但它最终得到相同的结果，并搜索在同一个表中连接两个外键以获得相同的主键，但我没有发现任何有用的东西。

```
 CREATE TABLE IF NOT EXISTS `Authors`.`Users` (
`userId` VARCHAR(100) NOT NULL,
`username` VARCHAR(25) NOT NULL,
`password` VARCHAR(16) NOT NULL,
`email` VARCHAR(254) NOT NULL,
`birthday` DATE NULL,
`aboutMe` TEXT(300) NOT NULL,
`facebookAccount` VARCHAR(25) NULL,
`twitterAccount` VARCHAR(25) NULL,
`linkedinAccount` VARCHAR(25) NULL,
`profileImage` VARCHAR(200) NULL,
PRIMARY KEY (`userId`),
UNIQUE INDEX `username_UNIQUE` (`username` ASC),
UNIQUE INDEX `email_UNIQUE` (`email` ASC))
ENGINE = InnoDB;
```

 CREATE TABLE IF NOT EXISTS `Authors`.`Books` (
`bookId` VARCHAR(100) NOT NULL,
`bookCategory` VARCHAR(25) NOT NULL,
`title` VARCHAR(25) NOT NULL,
`bookCover` VARCHAR(45) NOT NULL,
`bookDescription` VARCHAR(200) NOT NULL,
 `userId` VARCHAR(100) NOT NULL,
`price` DECIMAL(2,2) NOT NULL,
`introduction` VARCHAR(300) NOT NULL,
 PRIMARY KEY (`bookId`),
 INDEX `userId_idx` (`userId` ASC),
 CONSTRAINT `userId`
 FOREIGN KEY (`userId`)
 REFERENCES `Authors`.`Users` (`userId`)
 ON DELETE NO ACTION
 ON UPDATE NO ACTION)
 ENGINE = InnoDB;    
 ```
 ```
  CREATE TABLE IF NOT EXISTS `Authors`.`BooksReview` (
`bookId` VARCHAR(100) NOT NULL,
`rateMessage` VARCHAR(100) NULL,
`rateNumber` DECIMAL(1,1) NULL,
`userId` VARCHAR(100) NOT NULL,
 INDEX `userId_idx` (`userId` ASC),
 CONSTRAINT `bookId`
 FOREIGN KEY (`bookId`)
 REFERENCES `Authors`.`Books` (`bookId`)
 ON DELETE CASCADE
 ON UPDATE CASCADE,
 CONSTRAINT `userId`
 FOREIGN KEY (`userId`)
 REFERENCES `Authors`.`Users` (`userId`)
 ON DELETE CASCADE
 ON UPDATE CASCADE)
 ENGINE = InnoDB;
 ```
CREATE TABLE IF NOT EXISTS `Authors`.`Followers` (
`follower` VARCHAR(100) NOT NULL,
`followed` VARCHAR(100) NOT NULL,
 INDEX `follower_idx` (`follower` ASC),
 INDEX `followed_idx` (`followed` ASC),
 CONSTRAINT `follower`
 FOREIGN KEY (`follower`)
 REFERENCES `Authors`.`Users` (`userId`)
 ON DELETE CASCADE
 ON UPDATE CASCADE,
 CONSTRAINT `followed`
 FOREIGN KEY (`followed`)
 REFERENCES `Authors`.`Users` (`userId`)
 ON DELETE CASCADE
 ON UPDATE CASCADE)
ENGINE = InnoDB;

THIS IS THE QUERY
SELECT count(br.bookId) AS reviewsCount, count(b.bookId) AS booksCount, count(f.follower) AS followersCount
FROM Users AS u
LEFT JOIN Books AS b ON b.userId = u.userId
JOIN Followers AS f ON b.userId = f.followed AND f.follower = u.userId
INNER JOIN BooksReview AS br ON br.bookId = b.bookId 
                    AND b.bookId IN (SELECT bookId 
                                     FROM Books 
                                     WHERE userId = 'dbb21849-ccce-4af1-aa0f-6653919bf956');

我预计结果应该是 1 位追随者、4 本书和 4 条评论，但实际结果是 0。

DML：

Users->

       userId: dbb21849-ccce-4af1-aa0f-6653919bf956

       username: mostafabbbaron

ETC...

Books->

       userId: dbb21849-ccce-4af1-aa0f-6653919bf956

       bookId: 5f39c1ae-5e99-4b3a-8ee0-97a80c1ba9b1

ETC...

Followers->

       follower: dbb21849-ccce-4af1-aa0f-6653919bf956

       folllowed: b39c8e0c-4124-4339-8c30-e1fc8db5f2d4

ETC...

BooksReviews->

       userId: dbb21849-ccce-4af1-aa0f-6653919bf956

       bookId: aa44a455-dc28-476f-b4b9-47563a717f03

ETC...

标签： mysqljoinforeign-keysrelational-databasemysql-workbench

解决方案

无论哪种方式，您的查询都是错误的。但是结果为零的原因可能是这些条件：b.userId = u.userId和b.userId = f.followed AND f.follower = u.userId。

如果b.userId = u.userId然后b.userId = f.followed_f.followed = u.userId

如果f.followed = u.userId然后f.follower = u.userId_f.followed = f.follower

这意味着用户必须跟随他/她自己，我怀疑是这种情况。

我将按以下方式编写查询：

SELECT
count(DISTINCT b.bookId) AS booksCount,
count(br.bookId) AS reviewsCount,
(SELECT COUNT(*) FROM Followers AS f WHERE f.followed = u.userId) AS followersCount
FROM Users AS u
LEFT JOIN Books       AS b  ON b.userId  = u.userId
LEFT JOIN BooksReview AS br ON br.bookId = b.bookId 
WHERE u.userId = 'dbb21849-ccce-4af1-aa0f-6653919bf956'

注意：虽然它很好Users LEFT JOIN Books LEFT JOIN BooksReview，因为你有一个“关系链” Users <- Books <- BooksReview。但是您不应该只加入Followers表，因为它与Booksor没有真正的关系BooksReview，也不适合该链。这就是为什么我在 SELECT 子句中使用子查询来计算关注者的原因。

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