多个表上的SQL查询，AVG和COUNT

Question

我需要一个查询，我使用的查询因某种原因无法正常运行

以下是查询中涉及的所有表格。

以下是我想要的查询： 显示包含平均评分和推荐数量的图书清单

结果应该是这样的：

我已尝试过的内容：

SELECT        book.isbn, AVG(ratings.rating) AS [AVG Ratings], COUNT(recommend.isbn) AS [Number of recommendation]
FROM            book INNER JOIN
                         recommend ON book.isbn = recommend.isbn INNER JOIN
                         ratings ON book.isbn = ratings.isbn
GROUP BY book.isbn

但它不起作用，不知何故，AVG评级效果很好，但建议数没有，它与评级表冲突。

这是结果：

然而，当我单独尝试每一个时，一切都很有效：

对于AVG评级：

SELECT        book.isbn, AVG(ratings.rating) AS [AVG Ratings]
FROM            book INNER JOIN
                         ratings ON book.isbn = ratings.isbn
GROUP BY book.isbn

结果如下：

对于推荐＃：

SELECT        book.isbn, COUNT(recommend.isbn) AS [Number of recommendation]
FROM            book INNER JOIN
                         recommend ON book.isbn = recommend.isbn
GROUP BY book.isbn

结果如下：

所以我想要一个查询将两个视图合并为一个视图

Answer 1

如果您想获得准确的结果，那么您需要在加入之前进行聚合：

SELECT b.isbn, r.AvgRating, re.NumRecommendation
FROM  book b LEFT JOIN
      (SELECT r.isbn, AVG(r.rating) as AvgRating
       FROM rating r
       GROUP BY r.isbn
      ) r
      ON b.isbn = r.isbn LEFT JOIN
      (SELECT r.isbn, COUNT(*) as NumRecommendation
       FROM recommendation r
       GROUP BY r.isbn
      ) re
      on b.isbn = r.isbn ;

请注意，我也切换到了左外连接，因此您将获得所有书籍的结果，即使是那些缺少评分或推荐的书籍。

Answer 2

好吧，你可以将两个视图合并为一个..

在这里，这应该按你想要的方式工作

SELECT l.isbn, v.rating, r.rec
FROM book l, 
    (SELECT isbn, AVG(rating) AS rating FROM ratings GROUP BY isbn) v,
    (SELECT isbn, COUNT(isbn) AS rec FROM recommend GROUP BY isbn) r
WHERE l.isbn=v.isbn AND l.isbn=r.isbn

希望这有帮助。

祝你好运！

Answer 3

SELECT 
  book.isbn, 
  IFNULL(AVG(ratings.rating),"Not yet rated") AS [AVG Ratings],
  IFNULL(COUNT(DISTINCT recommend.iduse),0) AS [Number of recommendation]
FROM
  book
  LEFT JOIN recommend ON book.isbn = recommend.isbn 
  LEFT JOIN ratings ON book.isbn = ratings.isbn
GROUP BY book.isbn

应该诀窍：

• 计算推荐图书的不同用户将解决笛卡尔产品问题
• 左边加入通常的IFNULL()管道将使其适用于既没有推荐也没有评级的书籍

Answer 4

如果您使用的是SQL Server，则可以使用over子句：

SELECT 
  B.isbn, 
  AVG(ra.rating) OVER (PARTITION BY B.isbn) AS [AVG RATE],
  COUNT(re.isbn) OVER (PARTITION BY B.isbn) AS [RECOMMEND COUNT]
FROM Book B
LEFT JOIN recommend re ON B.isbn = re.isbn 
LEFT JOIN ratings ra ON B.isbn = ra.isbn