SQL查詢發現平均

Question

查找電影，他們的球迷都小於該特定類型

fan (fID: integer, fName: string) 
movies(mID: integer, mName: string, mGenre: string, mYear: integer) 
like(fID: integer, mID: integer) 

外鍵的平均數量的球迷：

(like.fID -> fan.fID) 
(like.mID-.> movie.mID) 

我試着這樣做是爲了獲得平均數量對於每部電影的球迷，它沒有工作：

SELECT mGenre, AVG(total) 
FROM findAverage, movies 
GROUP BY mGenre; 

CREATE VIEW findAverage AS 
SELECT m.mName, COUNT(DISTINCT L.fID) as total 
FROM like L, movies m 
WHERE L.mID = B.mID 
GROUP BY m.mName 

Answer 1

你需要改變你的看法，包括mId字段。那麼你可以用這個字段JOIN你的觀點回到movies表。

CREATE VIEW findAverage AS 
SELECT L.mId, m.mName, COUNT(DISTINCT L.fID) as total 
FROM like L 
    INNER JOIN movies m ON L.mID = B.mID 
GROUP BY L.mId, m.mName 

然後拿到平均每流派：

SELECT m.mGenre, AVG(f.total) 
FROM findAverage f 
    INNER JOIN movies m ON f.mId = m.mId 
GROUP BY m.mGenre