Question

我是Postgres的新手，我正在尝试连接表，获取平均值并按平均值排序。下面描述了两个表：

Table "public.movies"
   Column    |  Type   | Modifiers | Storage  | Stats target | Description 
-------------+---------+-----------+----------+--------------+-------------
 movieid     | integer | not null  | plain    |              | 
 title       | text    |           | extended |              | 
 releasedate | date    |           | plain    |              | 
 url         | text    |           | extended |              | 
 unknown     | integer |           | plain    |              | 
 action      | integer |           | plain    |              | 
 adventure   | integer |           | plain    |              | 
 animation   | integer |           | plain    |              | 
 children    | integer |           | plain    |              | 
 comedy      | integer |           | plain    |              | 
 crime       | integer |           | plain    |              | 
 documentary | integer |           | plain    |              | 
 drama       | integer |           | plain    |              | 



Table "public.ratings"
 Column  |  Type   | Modifiers | Storage | Stats target | Description 
---------+---------+-----------+---------+--------------+-------------
 userid  | integer | not null  | plain   |              | 
 movieid | integer | not null  | plain   |              | 
 rating  | integer |           | plain   |              | 
 time    | integer |           | plain   |              | 
Indexes:
    "ratings_pkey" PRIMARY KEY, btree (userid, movieid)
Foreign-key constraints:
    "ratings_movieid_fkey" FOREIGN KEY (movieid) REFERENCES movies(movieid)
Has OIDs: no

我必须按平均评分排序，因为一个movieid有来自不同用户的多个评级，但我必须按（asc）排序平均评分。我的查询如下，没有ORDER BY的情况下工作正常：

 SELECT (movies.title, AVG(ratings.rating)) 
 FROM movies 
    FULL JOIN ratings ON movies.movieid = ratings.movieid 
 WHERE (movies.comedy = 1) GROUP BY movies.title LIMIT 10;

如果我使用：ORDER BY AVG(ratings.rating)

，它会为所有电影评级返回1.00000000000

任何帮助都将非常感谢！在此先感谢：）

Answer 1

要按聚合顺序，您需要使用子查询，或者使用顺序位置作为订单说明符。

以下我有：

将您的FULL JOIN更改为LEFT OUTER JOIN - 如果没有movieid，则评分存在没有意义，如果有，则无效它会在此查询中显示为movieid null的平均评分。
添加ORDER BY 2告诉Pg按平均列排序

导致：

 SELECT movies.title, AVG(ratings.rating)
 FROM movies 
 LEFT OUTER JOIN ratings ON movies.movieid = ratings.movieid 
 WHERE (movies.comedy = 1)
 GROUP BY movies.title
 ORDER BY 2
 LIMIT 10

Postgres在AVERAGE函数上使用ORDER BY

1 个答案: