SQL查询发现平均

Question

查找电影，他们的球迷都小于该特定类型

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

外键的平均数量的球迷：

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

我试着这样做是为了获得平均数量对于每部电影的球迷，它没有工作：

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

你需要改变你的看法，包括mId字段。那么你可以用这个字段JOIN你的观点回到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 

然后拿到平均每流派：

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