将同一表中的两个foregin键联接在一起引用相同的主键

Question

我试图连接多个表（ BooksReview ， Followers ， Books ）与Users表，该表的Books引用userId从Users ， BooksReview引用userId这是userId谁审查一本书，并引用bookId从Books这是被审查书籍ID在最后，但并非最不重要的Followers表（这里我认为问题出在哪里）有从它具有相同的主键两个引用follower从userId在Users表， followed还从userId在Users表。

问题：我创建了一个MySQL查询，以根据特定用户获取特定图书的书评数量，并获取该用户的图书数量以及他/她拥有的Followers数量，但是当我添加Followers加入部分时在我的查询中，所有值显示0个结果，预期值为4本书，4条评论和1个关注者。

我曾尝试更改查询中的联接类型，但最终得到的结果相同，并在同一表中搜索联接两个foregin键以获取相同的主键，但是我没有发现任何有用的方法。

```
 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');

我希望结果应为1位关注者，4本书和4条评论，但实际结果为0。

DML：

Users ->

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

       username: mostafabbbaron

等等...

Books ->

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

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

等等...

Followers ->

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

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

等等...

BooksReviews ->

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

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

等等...

Answer 1

您的查询错误的任何一种方式。 但是，导致结果为零的原因可能是以下情况： b.userId = u.userId和b.userId = f.followed AND f.follower = u.userId 。

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

如果f.followed = u.userId和f.follower = u.userId则f.followed = f.follower

这意味着用户必须跟随他/她自己，我怀疑情况就是如此。

我将通过以下方式编写查询：

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：最好让Users LEFT JOIN Books LEFT JOIN BooksReview ，因为您有一个“关系链” Users <- Books <- BooksReview 。 但是您不应该仅加入Followers表，因为它与Books或BooksReview并没有真正的关系，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。