如何查询 COUNT 连接 table 和 return 记录同时计数 = 0 和计数 > 0

Question

我有一个产品 table 和一个评论 table。
我想对 return 每个产品的评论的 COUNT 和 AVG 进行查询。
AND 如果没有评论，我希望它 return 0/null for COUNT and AVG.

产品table

+--------+----------+
| prodId | prodName |
+--------+----------+
|   1    |  apple   |
|   2    |  banana  |
|   3    |  cacao   |
+--------+----------+

回顾table

+----------+----------+--------+
| reviewId |  prodId  | rating |
+----------+----------+--------+
|    1     |     1    |    1   |
|    2     |     1    |    1   |
|    3     |     2    |    5   |
|    4     |     2    |    5   |
+----------+----------+--------+

这是我想要的结果：

+--------+----------+--------+-------+
| prodId | prodName |   avg  | count |
+--------+----------+--------+-------+
|   1    |  apple   |    1   |   2   |
|   2    |  banana  |    5   |   2   |
|   3    |  cacao   |   null |   0   |
+--------+----------+--------+-------+

我能够获得有评论的产品的 COUNT 和 AVG

SELECT p.prodid, p.prodname, avg(r.stars), count(r.productid)
FROM products p 
INNER JOIN reviews r ON p.productid=r.productid 
GROUP BY p.productid

+--------+----------+--------+-------+
| prodId | prodName |   avg  | count |
+--------+----------+--------+-------+
|   1    |  apple   |    1   |   2   |
|   2    |  banana  |    5   |   2   |
+--------+----------+--------+-------+

我还可以获得没有评论的产品的 COUNT 和 AVG

SELECT p.prodid, p.prodname, avg(r.stars), count(r.stars)
FROM products p
LEFT OUTER JOIN reviews r 
ON (p.productid=r.productid) WHERE r.productid IS NULL
GROUP BY p.productid

+--------+----------+--------+-------+
| prodId | prodName |   avg  | count |
+--------+----------+--------+-------+
|   3    |  cacao   |   null |   0   |
+--------+----------+--------+-------+

但我不知道 MySql 是否有一种方法可以查询和统计在一个查询中匹配的结果和统计不匹配的结果。

Answer 1

只需从您的第二个查询中删除 where condition

SELECT p.prodid, p.prodname, avg(r.stars), count(r.stars)
FROM products p
LEFT OUTER JOIN reviews r 
ON p.productid=r.productid
GROUP BY p.productid,p.prodname

Answer 2

实际上 MySQL "LEFT JOIN" 与 LEFT OUTER JOIN 相同。 OUTER 关键字是可选的。要获得结果，您只需要执行简单的 "LEFT JOIN" 并按 ID（或任何其他唯一键）分组：

SELECT 
    `p`.`prodId` AS `ID`, 
    `p`.`prodName` AS `Product`, 
    avg(`r`.`rating`) AS `AVG Rating`, 
    count(`r`.`reviewId`) AS `Vote Count`
FROM `products` AS `p`
LEFT JOIN `reviews` AS `r` ON `p`.`prodId` = `r`.`prodId`
GROUP BY `p`.`prodId`

如何查询 COUNT 连接 table 和 return 记录同时计数 = 0 和计数 > 0

How to Query for COUNT on a join table and return records with BOTH count=0 AND count>0

mysql

jointable