Mysql 查找每个国家评分最高的图书。如果有平局 "print" 评分最高的书

Question

我有以下表格：

CREATE TABLE `country` (
  `name` VARCHAR(60) NOT NULL,
  `code` VARCHAR(3) UNIQUE NOT NULL,
  PRIMARY KEY  (`code`)
);

CREATE TABLE `user` (
  `userId` INT UNIQUE NOT NULL AUTO_INCREMENT,
  `country` VARCHAR(3) NOT NULL,
  `age` INT NOT NULL,
  PRIMARY KEY  (`userId`),
  CONSTRAINT `fk_user_country` FOREIGN KEY (`country`) REFERENCES `country`(`code`)
);

CREATE TABLE `bookRating` (
  `userId` INT NOT NULL,
  `isbn` VARCHAR(13) NOT NULL,
  `rate` INT NOT NULL,
  `date` DATE NOT NULL, 
  CONSTRAINT `fk_bookRating_user` FOREIGN KEY (`userId`) REFERENCES `user`(`userId`),
  CONSTRAINT `fk_bookRating_book` FOREIGN KEY (`isbn`) REFERENCES `book`(`isbn`)
);

CREATE TABLE `book` (
  `isbn` varchar(13) UNIQUE NOT NULL,
  `bookTitle` VARCHAR(280),
  `bookAuthor` VARCHAR(150),
  `yearPublication` int(4),
  -- `yearPublication` must be an integer because we have value less that 1901 in dataset
  `publisher` VARCHAR(135),
  PRIMARY KEY  (`isbn`),
  CONSTRAINT `publication_yea_chk` check ((`yearPublication` > -1) && (`yearPublication` < 2101))
);

正如我在标题上所说的，我想为每个国家/地区找到平均评分最高的书

我试过这个查询：

select T1.name, T1.BookTitle, Rate
from    
(
    select C.Code, AVG(BR.rate) MAXRating
    from   `bookRating` BR
            inner join `book` B on BR.isbn = B.isbn
            INNER JOIN `USER` U ON BR.UserID = U.USERId
            INNER JOIN `COUNTRY` C ON U.country = C.Code
    group by C.Code
) T
inner join 
(
    select C.Code, C.name, B.BookTitle, BR.ISBN, BR.rate
    from   `bookRating` BR
            inner join `book` B on BR.isbn = B.isbn
            INNER JOIN `USER` U ON BR.UserID = U.USERId
            INNER JOIN `COUNTRY` C ON U.country = C.Code
) T1 ON T.Code = T1.Code AND T.MAXRATING = T1.RATE;

我很确定这行得通。但我想让它像如果 2 本书或更多书的平均评分相同，我想要评分最高的那本书。

我想我可以使用 If() 语句，但我怎么能 If(... , a condition)

我该怎么做？

UPDATE

我已经创建了数据库并在 db fidle 中插入了一些信息： https://www.db-fiddle.com/f/s6wKhKhxXMX1W2x9VZn9da/1

Answer 1

您可以加入表格，按国家和书籍聚合以获得所有平均评分，并对聚合结果使用 window 函数 MAX() 和 FIRST_VALUE() 以获得书籍每个国家的平均水平最高：

SELECT DISTINCT c.name, 
       FIRST_VALUE(b.bookTitle) OVER (
         PARTITION BY c.Code
         ORDER BY AVG(r.rate) DESC, COUNT(*) DESC
       ) bookTitle,
       MAX(AVG(r.rate)) OVER () AverageRating
FROM country c
INNER JOIN users u ON u.country = c.Code
INNER JOIN bookRating r ON r.UserID = u.UserID
INNER JOIN book b ON b.isbn = r.isbn
GROUP BY c.Code, b.isbn;

参见demo。

Mysql 查找每个国家评分最高的图书。如果有平局 "print" 评分最高的书

Mysql find the book with the highest rating for each country. If there is a tie "print" the book with the highest number of ratings

mysql

select

join

group-by

window-functions