每組選擇一行？

Question

我有一個表叫收視率，看起來像這樣

 
Rating | Picid | User 
    8   12   6 
    7   12   6 
    9   15   7 
    5   16   7 
    9   17   8 
    10   2   8 
    7   18   3 
    10   22   12 

我想組基礎上，picid的收視率，但只能選擇每用戶評價最高的picid。我有這樣已經查詢：

SELECT *, AVG(rating) AS total FROM ratings 
    GROUP BY picid ORDER by total DESC 

眼下查詢將輸出這樣的信息：

 
Rating | Picid | User 
7.5   12   6 
    9   15   7 
    5   16   7 
    9   17   8 
    10   2   8 
    7   18   3 
    10   22   12 

所以它的基礎上的picid成功羣體的收視率，但我只需要每個用戶最高的一個。我希望它這樣的輸出信息：

 
Rating | Picid | User 
7.5   12   6 
    9   15   7 
    10   2   8 
    7   18   3 
    10   22   12 

怎麼看呢只允許每個用戶的收視率最高的picid？ 如何修改我的查詢來執行此操作？

讓我知道你是否需要更多關於我所問的問題的解釋。

Answer 1

你首先必須計算的平均收視爲每個(Picid,User)組合，然後找到group-wise maximum在該物化結果：

SELECT Rating, PicID, User 
FROM (
    SELECT AVG(Rating) AS Rating, Picid, User 
    FROM  ratings 
    GROUP BY Picid, User 
) t1 NATURAL JOIN (
    SELECT MAX(Rating) AS Rating, User 
    FROM  (
    SELECT AVG(Rating) AS Rating, User 
    FROM  ratings 
    GROUP BY Picid, User 
) t 
    GROUP BY User 
) t2 

看到它的sqlfiddle。

Answer 2

我不認爲這是最好的方式，但它的工作原理：

SELECT MAX(total) as maxTotal, picid, User 
FROM 
    (SELECT *, AVG(rating) AS total 
    FROM ratings 
    GROUP BY picid 
    ORDER BY total DESC) as a 
GROUP BY User 
ORDER BY maxTotal desc; 

您可以使用此sqlfiddle嘗試其他。