sql - SQL Compact Edition LEFT OUTER JOIN 平均

Question

我有一张带有产品的表格和一张带有评级的表格。我有以下查询；

SELECT products.name, ratings.rating 
FROM   products 
       LEFT OUTER JOIN ratings 
       ON products.id = ratings.product_id;

分别返回给我一个很好的行列表，其中包含产品名称和每个评级。

我试图实现它返回一行，显示产品名称及其平均评分。似乎无法找出如何。

提前致谢。

Answer 1

您想查看GROUP BY 语句。例子：

SELECT products.name AS Product_Name, 
       AVG(ratings.rating) AS Average_Rating
FROM   products 
       LEFT OUTER JOIN ratings 
       ON products.id = ratings.product_id
GROUP BY products.name
ORDER BY products.name;

---

在使用 GROUP BY 语句时，您可能希望根据一个或多个聚合函数的结果过滤结果集。您使用HAVING 语句来执行此操作：

SELECT products.name AS Product_Name, 
       AVG(ratings.rating) AS Average_Rating
FROM   products 
       LEFT OUTER JOIN ratings 
       ON products.id = ratings.product_id
GROUP BY products.name
HAVING AVG(ratings.rating) >= 3
ORDER BY products.name;