什么不对这个SQL

Question

我有以下SQL查询：

SELECT tblBooks.bookID, 
    tblBooks.title, 
    tblBooks.author, 
    tblBooks.coverImage, 
    ROUND(ROUND(tblReviews.rating * 2)/2, 1) AS rating 
FROM tblBooks 
    LEFT JOIN tblReviews 
     ON tblBooks.bookID = tblReviews.bookID 
     LEFT JOIN tblMembers 
      ON tblReviews.userID = tblMembers.userID 
WHERE tblReviews.bookID IS NOT NULL 
ORDER BY rating DESC 
LIMIT 0, 40 

我想查询数据库，并返回的平均等级，所以如果一本书进行了审查6次，每次评分共达来自6位用户并计算出平均值。现在只有最新的评级被返回。当我将此行更改为：

ROUND(ROUND(AVG(tblReviews.rating) * 2)/2, 1) AS rating 

只有一个结果总共返回，所以显然有些问题，但我不知道是什么。

如果任何人都可以对此有所了解，我认为这将与我的加入有关。

Answer 1

您需要使用group by这样的：

SELECT tblBooks.bookID, tblBooks.title, tblBooks.author, tblBooks.coverImage, 
     AVG(tblReviews.rating) as avgRating 
FROM tblBooks 
    LEFT JOIN tblReviews 
    ON tblBooks.bookID = tblReviews.bookID 
WHERE tblReviews.bookID IS NOT NULL 
group by tblBooks.bookID, tblBooks.title, tblBooks.author, tblBooks.coverImage 
ORDER BY avgrating DESC 

您的查询似乎不使用从成员表中的任何信息，所以我淘汰了。它甚至不基于它进行过滤，因为您使用的是left join。

Answer 2

我认为你缺少的是

GROUP BY tblBooks.bookID, tblBooks.title, tblBooks.author, etc... 

AVG是一个聚合函数，总结出的数据，使用GROUP BY创建聚集的群体。

Answer 3

试试这个：

... 
WHERE tblReviews.bookID IS NOT NULL 
GROUP BY tblBooks.bookID 
ORDER BY rating DESC 
LIMIT 0, 40 

干杯。

Answer 4

为什么不使用AVG代替？

SELECT tblBooks.bookID, 
    tblBooks.title, 
    tblBooks.author, 
    tblBooks.coverImage, 
    AVG(tblReviews.rating) AS rating 
FROM tblBooks 
    LEFT JOIN tblReviews 
     ON tblBooks.bookID = tblReviews.bookID 
     LEFT JOIN tblMembers 
      ON tblReviews.userID = tblMembers.userID 
WHERE tblReviews.bookID IS NOT NULL 
Group By tblBooks.bookID, tblBooks.title, tblBooks.author, tblBooks.coverImage 
ORDER BY rating DESC 
LIMIT 0, 40 

FYI

在MySQL中，你没有全部虽然这是一个很好的做法，因为你需要使用其他关系数据库管理系统时，需要按所有列。

Answer 5

你也不需要WHERE。你基本上是做一个INNER加入：

SELECT t.bookID, 
     t.title, 
     t.author, 
     t.coverImage, 
     AVG(r.rating) AS rating 
FROM tblBooks AS t 
    JOIN tblReviews AS r 
     ON b.bookID = r.bookID 
GROUP BY t.bookID 
--  , t.title, t.author, t.coverImage 
ORDER BY rating DESC 
LIMIT 40 ; 

注：只使用GROUP BY t.bookID（并且没有其他列在那里）会给出正确的结果只有bookID是主键（或具有唯一约束）在tblBooks。所有其他DBMS不允许这样做（Postgres除外，但该产品已正确实现该功能，并且确实检查其他列是否依赖于分组列）。 MySQL根本不检查，所以在很多情况下它可能会导致错误的结果。

这也取决于sql_mode的设置，上述是否会无误地运行。

---

您可以选择先按，然后加入。这是有效的SQL，并且不会出现错误结果：

SELECT t.bookID, 
     t.title, 
     t.author, 
     t.coverImage, 
     g.rating 
FROM tblBooks AS t 
    JOIN 
     (SELECT r.bookID, 
       AVG(r.rating) AS rating 
      FROM tblReviews AS r 
      GROUP BY r.bookID 
     ) AS g 
     ON b.bookID = r.bookID 
ORDER BY rating DESC 
LIMIT 40 ;