PostgreSQL - 从连接表中选择聚合列并使用它来求和

Question

我想选择从三个表中聚合的字段，并为帖子输出三个字段，postId、calculatedRate 和 count，我试过但没有用

目前我有这个示例查询只得到count 和postId

select
    mr.count,
    mr.postId
from
    (
        select
            "m"."postId" as postId,
            count("m"."postId") as count
        from
            "Rating" r
        left join "Messenger" m on
            "r"."id" = "m"."ratingId"
        group by
            "m"."postId"
    ) mr
group by
    mr.postId,
    mr.count

Messenger（帖子和评级的映射表）

id	postId	ratingId
1	1	4
2	1	5
3	2	6

发帖

id	title
1	post one
2	post two

评分

id	a	b
4	5.0	5.0
5	3.0	3.0
6	2.0	4.0

预期结果（calculatedRate = 同一帖子 id 内的平均值（（rating.a + rating.b）/ 2））

id	postId	calculatedRate	count
1	1	4.0	2
2	2	3.0	1

Answer 1

我不确定我是否得到了你要找的东西，但如果你想要每个 MESSENGER.ID 的 COUNT 和 CALCULATEDRATE，那么下面应该这样做

SELECT 
MESSENGER.ID,
POST.ID POSTID, 
COUNT(*) NR_RATINGS,
(RATING.A + RATING.B)/2 CALCULATEDRATE
FROM 
RATING JOIN MESSENGER ON RATING.ID = MESSENGER.RATINGID
JOIN POST ON MESSENGER.POSTID=POST.ID

编辑：如果您想要平均邮寄，您可以通过以下方式实现这一目标

SELECT 
POST.ID POSTID, 
COUNT(*) NR_RATINGS,
avg((RATING.A + RATING.B)/2) CALCULATEDRATE
FROM 
RATING JOIN MESSENGER ON RATING.ID = MESSENGER.RATINGID
JOIN POST ON MESSENGER.POSTID=POST.ID
GROUP BY POST.ID