SQL - ordering table by information from multiple tables - sql

Question

Title of the question may not have been very clear - I am not really sure how to name this question, but I hope that my explanation will make my problem clearer.
I have 3 tables:
[1] score
id
rating_type
1
UPVOTE
2
UPVOTE
3
DOWNVOTE
4
UPVOTE
5
DOWNVOTE
6
DOWNVOTE
[2] post_score
post_id
score_id
1
1
1
2
1
3
2
4
2
5
2
6
and [3] post
id
title
1
title1
2
title2
My goal is to order [3] post table by score.
Assume UPVOTE represents value of 1 and DOWNVOTE value of -1; In this example, post where id = 1 has 3 scores related to it, and the values of them are UPVOTE, UPVOTE, DOWNVOTE, making the "numeric score" of this post: 2;
likewise, post where id = 2, also has 3 scores, and those values are: UPVOTE, DOWNVOTE, DOWNVOTE, making the "numeric score": -1;
How would I order post table by this score? In this example, if I ordered by score asc, I would expect the following result:
id
title
2
title2
1
title1
My attempts didn't go far, I am stuck here with this query currently, which doesn't really do anything useful yet:
WITH fullScoreInformation AS (
SELECT * FROM score s
JOIN post_score ps ON s.id = ps.score_id),
upvotes AS (SELECT * FROM fullScoreInformation WHERE rating_type = 'UPVOTE'),
downvotes AS (SELECT * FROM fullScoreInformation WHERE rating_type = 'DOWNVOTE')
SELECT p.id, rating_type, title FROM post p JOIN fullScoreInformation fsi on p.id = fsi.post_id
I am using PostgreSQL. Queries will be used in my Spring Boot application (I normally use native queries).
Perhaps this data structure is bad and I should have constructed my entities differently ?

Answer 1

My goal is to order post table by score. Assume UPVOTE represents value of 1 and DOWNVOTE value of -1
One option uses a subquery to count the upvotes and downvotes of each post:
select p.*, s.*
from post p
cross join lateral (
select
count(*) filter(where s.rating_type = 'UPVOTE' ) as cnt_up,
count(*) filter(where s.rating_type = 'DOWNVOTE') as cnt_down
from post_score ps
inner join score s on s.id = ps.score_id
where ps.post_id = p.id
) s
order by s.cnt_up - s.cnt_down desc
Perhaps this data structure is bad and I should have constructed my entities differently ?
As it stands, I don't see the need for two distinct tables post_score and score. For the data you have showed, this is a 1-1 relationship, so just one table should be sufficient, storing the post id and the rating type.

Answer 2

You better use a LEFT join, otherwise you wouldn't get posts that have no votes yet. Then aggregate to get the fitered sum of the scores. Then add these sums, apply coalesce() to get 0 for posts without votes and order by the result.
SELECT p.id,
p.title
FROM post p
LEFT JOIN post_score ps
ON ps.post_id = p.id
LEFT JOIN score s
ON s.id = ps.score_id
GROUP BY p.id,
p.title
ORDER BY coalesce(sum(1) FILTER (WHERE rating_type = 'UPVOTE')
+
sum(-1) FILTER (WHERE rating_type = 'DOWNVOTE'),
0);
I second GMB's comment about the superfluous table.