在线商店建模,用户可以在其中购买商品并对他们购买的商品进行评分。有四张表
购买(用户,商品)
用户(用户ID,名称)
价格(项目、评级)
商品(商品ID、描述、价格)
我想向每个用户返回一份他们未购买但平均评分最高的商品列表。
到目前为止我构建的代码有点笨拙,但似乎完成了一半的工作:
SELECT U.UserID, U.name, I.ItemID, I.Description, AVG(R.Rating) AS avgRate
FROM Item I, User U, Rates R
WHERE R.Item=I.ItemID
AND NOT EXISTS
(SELECT Item FROM Buys B WHERE I.ItemID = B.item and U.UserID = B.user)
group by U.UserID, item,I.ItemID Order by U.UserID, avgRate DESC
这给了我以下输出:
UserID name ItemID Decription avgRate
1 Jones 5 Computing Textbook 5
1 Jones 11 Tennis Ball 3.5
1 Jones 12 Tennis Raquet 4
2 Brown 5 Computing Textbook 5
2 Brown 11 Tennis Ball 3.5
2 Brown 12 Tennis Raquet 4
但我希望通过输出只为每个用户选择一项:
UserID name ItemID Decription avgRate
1 Jones 5 Computing Textbook 5
2 Brown 5 Computing Textbook 5
我尝试过限制排序,但它只给了我 1 行,而且你不能将 MAX() 函数与 AVG 一起使用...
最佳答案
假设您可能使用的同一用户的两种产品的最高平均值永远不会相同:
with cte1 (UserID,name,ItemID,Decription,avgRate)
as
(
SELECT U.UserID, U.name, I.ItemID, I.Description, AVG(R.Rating) AS avgRate
FROM Item I, User U, Rates R
WHERE R.Item=I.ItemID
AND NOT EXISTS
(SELECT Item FROM Buys B WHERE I.ItemID = B.item and U.UserID = B.user)
group by U.UserID, item,I.ItemID Order by U.UserID, avgRate DESC
)
,
cte2 (UserID,name,ItemID,Decription,avgRate,rank1) as
(
select * , rank() over (partition by name order by avgrate)from cte1
)
select UserID,name,ItemID,Decription,avgRate from cte2 where rank1 in
(
select max(rank1) from cte2 group by userid
)
order by userid
关于mysql - SQL 分组项的最大平均值,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28986686/