如何按用户价格订购书籍

【问题标题】：How to order books by users rate如何按用户价格订购书籍
【发布时间】：2019-08-29 01:34:56
【问题描述】：

我正在尝试按用户评分描述检索所有书籍：

我将费率保存在中间表 BooksRates 中，因为同一本书会有多个费率，现在我想在一个查询中从数据库中检索前 10 部评分最高的书，其中也包含费率。

例如：

哈利波特 4.5 率

SELECT T1.*, T2.AVG(T2.Rate) AS Rate
    FROM Books AS T1 
    INNER JOIN BookRates as T2
    ON T1.BookId = T2.BookId
    GROUP BY T1.BookId
    ORDER BY Rate desc

此查询未返回所需的结果。

【问题讨论】：

标签： sql-server select sql-order-by

【解决方案1】：

首先，您使用group by 获得每个BookId 的平均Rate，然后加入Books 以获取每本书的详细信息：

SELECT b.*, g.Rate FROM Books AS b INNER JOIN ( 
    SELECT T1.BookId, AVG(T2.Rate) AS Rate
    FROM Books AS T1 
    INNER JOIN BookRates as T2
    ON T1.BookId = T2.BookId
    GROUP BY T1.BookId
) AS g on g.BookId = b.BookId
ORDER BY g.Rate desc

如果有书籍没有评分，那么你必须使用左连接：

SELECT b.*, coalesce(g.Rate, 0) AS Rate FROM Books AS b LEFT JOIN ( 
    SELECT T1.BookId, AVG(T2.Rate) AS Rate
    FROM Books AS T1 
    INNER JOIN BookRates as T2
    ON T1.BookId = T2.BookId
    GROUP BY T1.BookId
) AS g on g.BookId = b.BookId
ORDER BY coalesce(g.Rate, 0) desc

【讨论】：