Postgres: Many to many joins creates double output - sql

Question

I've recently added a many to many JOIN to one of my queries to add a "tag" functionality. The many to many works great, however, it's now causing a previously working part of the query to output records twice.
SELECT v.*
FROM "Server" AS s
JOIN "Vote" AS v ON (s.id = v."serverId")
JOIN "_ServerToTag" st ON (s.id = st."A")
OFFSET 0 LIMIT 25;
id | createdAt | authorId | serverId
-----+-------------------------+----------+----------
190 | 2020-12-23 15:47:25.476 | 6667 | 3
190 | 2020-12-23 15:47:25.476 | 6667 | 3
194 | 2020-12-21 15:47:25.476 | 6667 | 3
194 | 2020-12-21 15:47:25.476 | 6667 | 3
In the example above:
Server is my main table which contains a bunch of entries. Think of it as Reddit Posts, they have a title, content and use the Vote table to count "upvotes".
id | title
----+-------------------------------
3 | test server 3
Votes is a really simple table, it contains a timestamp of the "upvote", who created it, and the Server.id it is assigned to.
_ServerToTag is a table that contains two columns A and B. It connects Server to another table which contains Tags.
A | B
---+---
3 | 1
3 | 2
The above is a much-simplified query, in reality, I am suming the outcome of the query to get a number total of Votes.
The desired outcome would be that the results are not duplicated:
id | createdAt | authorId | serverId
-----+-------------------------+----------+----------
190 | 2020-12-23 15:47:25.476 | 6667 | 3
194 | 2020-12-21 15:47:25.476 | 6667 | 3
I'm really unsure why this is even happening so I have absolutely no idea how to fix it.
Any help would be greatly appreciated.
Edit:
DISTINCT works if I want to query the Vote table. But not in more complex queries. In my case it would look something more like this:
SELECT s.id, s.title, sum(case WHEN v."createdAt" >= '2020-12-01' AND v."createdAt" < '2021-01-01'
THEN 1 ELSE 0 END ) AS "voteCount",
FROM "Server" AS s
LEFT JOIN "Vote" AS v ON (s.id = "serverId")
LEFT JOIN "_ServerToTag" st ON (s.id = st."A");
id | title | voteCount
----+-------------------------------+-----------
3 | test server 3 | 4
In the above, I only need the voteCount column to be DISTINCT.
SELECT s.id, s.title, sum(DISTINCT case WHEN v."createdAt" >= '2020-12-01' AND v."createdAt" < '2021-01-01'
THEN 1 ELSE 0 END ) AS "voteCount",
FROM "Server" AS s
LEFT JOIN "Vote" AS v ON (s.id = "serverId")
LEFT JOIN "_ServerToTag" st ON (s.id = st."A");
id | title | voteCount
----+-------------------------------+-----------
3 | test server 3 | 1
The above kind of works, but it seems to only count one vote even if there are multiple.

Answer 1

It appears that the problem is that you added the join to _ServerToTag. Because there are multiple rows in _ServerToTag for each row in Server the query returns multiple rows for each server, one for each matching row in _ServerToTag.
It appears that _ServerToTag was adde to the query so it will only include servers which have tags. If that's your intent you can use:
SELECT v.id, v.authorId, v.serverId, COUNT(DISTINCT v.createdAt) AS TOTAL_VOTES
FROM "Server" AS s
INNER JOIN "Vote" AS v
ON s.id = v."serverId"
INNER JOIN (SELECT DISTINCT "A" FROM "_ServerToTag") st
ON s.id = st."A"
WHERE v."createdAt" >= '2020-12-01' AND
v."createdAt" < '2021-01-01'
GROUP BY v.id, v.authorId, v.serverId
OFFSET 0 LIMIT 25
or
SELECT v.id, v.authorId, v.serverId, COUNT(DISTINCT v.createdAt) AS TOTAL_VOTES
FROM "Server" AS s
INNER JOIN "Vote" AS v
ON s.id = v."serverId"
WHERE v."createdAt" >= '2020-12-01' AND
v."createdAt" < '2021-01-01' AND
s.id IN (SELECT "A" FROM "_ServerToTag")
GROUP BY v.id, v.authorId, v.serverId
OFFSET 0 LIMIT 25
which may communicate the intent of the query a bit better.
EDIT
If you want to be able to count entries which have no votes you'll need to use an outer join to pull in the (potentially non-existent) votes and then use a CASE expression to only count votes if they exist:
SELECT s.id, v.id, v.authorId, v.serverId,
CASE
WHEN v.id IS NULL THEN 0
ELSE COUNT(DISTINCT v.createdAt)
END AS TOTAL_VOTES
FROM "Server" AS s
LEFT OUTER JOIN "Vote" AS v
ON s.id = v."serverId"
WHERE v."createdAt" >= '2020-12-01' AND
v."createdAt" < '2021-01-01' AND
s.id IN (SELECT "A" FROM "_ServerToTag")
GROUP BY s.id, v.id, v.authorId, v.serverId
OFFSET 0 LIMIT 25
You may not actually need that though - you may be able to get away with
SELECT s.id, v.id, v.authorId, v.serverId,
COUNT(DISTINCT v.createdAt) AS TOTAL_VOTES
FROM "Server" AS s
LEFT OUTER JOIN "Vote" AS v
ON s.id = v."serverId"
WHERE v."createdAt" >= '2020-12-01' AND
v."createdAt" < '2021-01-01' AND
s.id IN (SELECT "A" FROM "_ServerToTag")
GROUP BY s.id, v.id, v.authorId, v.serverId
OFFSET 0 LIMIT 25

Answer 2

Okay so I went and asked a friend for help after not really being able to fix my problem with the answers I received.
I think my query was just too complex and confusing and I was suggested to use subqueries to make it less complicated and easier to manage.
My query now looks like this:
SELECT
s.id
, s.title
, COALESCE(v."VOTES", 0) AS "voteCount"
FROM "Server" AS s
-- Join tags
INNER JOIN
(
SELECT
st."A"
, json_agg(
json_build_object(
'id',
t.id,
'tagName',
t."tagName"
)
) as "tagsArray"
FROM
"_ServerToTag" AS st
INNER JOIN
"Tag" AS t
ON
t.id = st."B"
GROUP BY
st."A"
) AS tag
ON
tag."A" = s.id
-- Count votes
LEFT JOIN
(
SELECT
"serverId"
, COUNT(*) AS "VOTES"
FROM
"Vote" as v
WHERE
v."createdAt" >= '2020-12-01' AND
v."createdAt" < '2021-01-01'
GROUP BY "serverId"
) as v
ON
s.id = v."serverId"
OFFSET 0 LIMIT 25;
This works exactly the same way but by selecting what I need directly in the joins it's more readable and I have more control over the data I get back.