Question

我正在尝试按用户对desc的评分来检索所有图书：

我将费率保留在中间表BooksRates中，因为同一本书将有多个费率，现在我想在一个查询中同时从数据库中检索前10个评分最高的图书，并且还包含费率。

例如：

哈利·波特4.5费率


def input_output_decorator(preprocess=None, postprocess=None):
    def decorator(func):
        if preprocess and postprocess:
            def func_wrapper(*args, **kwargs):
                return postprocess(func(preprocess(*args, **kwargs)))
        elif preprocess:
            def func_wrapper(*args, **kwargs):
                return func(preprocess(*args, **kwargs))
        elif postprocess:
            def func_wrapper(*args, **kwargs):
                return postprocess(func(*args, **kwargs))
        else:  
            return func

        return wraps(func)(func_wrapper)

    return decorator

此查询未返回所需结果。

Answer 1

首先，您将得到Rate中每本BookId的平均group by，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

如果某些图书未得到评级，则必须使用左联接：

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

如何按用户等级订购书籍

1 个答案: