MySQL AVG() 如果为 NULL 则返回 0

Question

我有 3 个表，如下所示:

mysql> select * from Raccoon;
+----+------------------+----------------------------------------------------------------------------------------------------+
| id | name             | image_url                                                                                          |
+----+------------------+----------------------------------------------------------------------------------------------------+
|  3 | Jesse Coon James | http://www.pawfun.com/wp/wp-content/uploads/2010/01/rabbid.png                                     |
|  4 | Bobby Coon       | https://c2.staticflickr.com/6/5242/5370143072_dee60d0ce2_n.jpg                                     |
|  5 | Doc Raccoon      | http://images.encyclopedia.com/utility/image.aspx?id=2801690&imagetype=Manual&height=300&width=300 |
|  6 | Eddie the Rac    | http://www.felid.org/jpg/EDDIE%20THE%20RACCOON.jpg                                                 |
+----+------------------+----------------------------------------------------------------------------------------------------+
4 rows in set (0.00 sec)

mysql> select * from Review;
+----+------------+-------------+---------------------------------------------+--------+
| id | raccoon_id | reviewer_id | review                                      | rating |
+----+------------+-------------+---------------------------------------------+--------+
|  1 |          3 |           1 | This raccoon was a fine raccoon indeed.     |      5 |
|  2 |          5 |           2 | This raccoon did not do much for me at all. |      2 |
|  3 |          3 |           1 | asdfsadfsadf                                |      5 |
|  4 |          5 |           2 | asdfsadf                                    |      1 |
+----+------------+-------------+---------------------------------------------+--------+
4 rows in set (0.00 sec)

mysql> select * from Reviewer;
+----+---------------+
| id | reviewer_name |
+----+---------------+
|  1 | Kane Charles  |
|  2 | Cameron Foale |
+----+---------------+
2 rows in set (0.00 sec)

我正在尝试构建一个选择查询，该查询将返回 Raccoon 中的所有列以及一个获取平均 Review.rating 的额外列(按 id 分组)。我面临的问题是，不能保证每个 Raccoon 的 Review 表中都会有行(由 FK 确定，raccoon_id 引用 Raccoon.id。在 Review 表中存在零行的情况下(对于给定的 Raccoon.id，即 Review.raccoon_id)，我希望查询返回 0 作为该浣熊的平均值。

下面是我正在使用的当前查询:

mysql> SELECT *, (SELECT IFNULL(AVG(rating),0) FROM Review WHERE raccoon_id=Raccoon.id GROUP BY raccoon_id) AS "AVG" FROM Raccoon ORDER BY "AVG" ASC;
+----+------------------+----------------------------------------------------------------------------------------------------+--------+
| id | name             | image_url                                                                                          | AVG    |
+----+------------------+----------------------------------------------------------------------------------------------------+--------+
|  3 | Jesse Coon James | http://www.pawfun.com/wp/wp-content/uploads/2010/01/rabbid.png                                     | 5.0000 |
|  4 | Bobby Coon       | https://c2.staticflickr.com/6/5242/5370143072_dee60d0ce2_n.jpg                                     |   NULL |
|  5 | Doc Raccoon      | http://images.encyclopedia.com/utility/image.aspx?id=2801690&imagetype=Manual&height=300&width=300 | 1.5000 |
|  6 | Eddie the Rac    | http://www.felid.org/jpg/EDDIE%20THE%20RACCOON.jpg                                                 |   NULL |
+----+------------------+----------------------------------------------------------------------------------------------------+--------+
4 rows in set (0.00 sec)

正如您在上面看到的，对于 ID 为 4 和 6 的 Raccoon，查询不会返回 0，它只是返回 NULL。我需要它返回如下内容(注意顺序，首先按最低平均评论排序):

+----+------------------+----------------------------------------------------------------------------------------------------+--------+
| id | name             | image_url                                                                                          | AVG    |
+----+------------------+----------------------------------------------------------------------------------------------------+--------+
|  4 | Bobby Coon       | https://c2.staticflickr.com/6/5242/5370143072_dee60d0ce2_n.jpg                                     | 0.0000 |
|  6 | Eddie the Rac    | http://www.felid.org/jpg/EDDIE%20THE%20RACCOON.jpg                                                 | 0.0000 |
|  5 | Doc Raccoon      | http://images.encyclopedia.com/utility/image.aspx?id=2801690&imagetype=Manual&height=300&width=300 | 1.5000 |
|  3 | Jesse Coon James | http://www.pawfun.com/wp/wp-content/uploads/2010/01/rabbid.png                                     | 5.0000 |
+----+------------------+----------------------------------------------------------------------------------------------------+--------+

Answer 1

在你的子查询之外使用 IFNULL 因为它会返回 null 如果在外表上没有匹配，

IFNULL((SELECT AVG(rating) FROM Review WHERE raccoon_id=Raccoon.id GROUP BY raccoon_id), 0) AS "AVG"

或者你也可以使用LEFT JOIN,

SELECT  ra.id, ra.name, ra.image_url,
        IFNULL(AVG(rv.rating),0)AS "AVG" 
FROM    Raccoon ra
        LEFT JOIN Review rv
            ON rv.raccoon_id = ra.id
GROUP   BY ra.id, ra.name, ra.image_url  
ORDER   BY "AVG" ASC;