PostgreSQL - select aggregated column from joined table and use it to sum

Question

I want to select fields that aggregated from three tables and output three fields for the post, postId, calculatedRate and count, i tried but not working

currently i have this sample query just get count and 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 (mapping table for Post and Rating)

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

Post

id	title
1	post one
2	post two

Rating

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

Expected result (calculatedRate = average within same post id((rating.a + rating.b) / 2))

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

Answer 1

I'm not sure I got what you're looking for, but if you want the COUNT and CALCULATEDRATE for each MESSENGER.ID then the following should do it

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

Edit: if you want the average by post you can achieve that with the following

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