CASE will ignore duplicate entries - mysql

Question

I have a table which store user votes, something like this:
+----+---------+-------+---------+
| id | post_id | value | user_id |
+----+---------+-------+---------+
| 1 | 103 | 1 | 1 |
| 2 | 105 | 1 | 3 |
| 3 | 106 | 1 | 1 |
| 4 | 108 | 0 | 1 |
| 5 | 108 | 0 | 2 |
| 6 | 105 | 0 | 2 |
| 7 | 105 | 1 | 1 |
+----+---------+-------+---------+
Where id is Vote ID, post_id is Post ID, value is a boolean for like/unlike (1 = Like, 0 = Unlike) and user_id is Voter ID.
I want to get a list which has:
All posts
Each post's like count
Each post's unlike count
Vote ID of where a user (for example, user 1) voted. So I can update the value later
The vote that user gave
So I wrote this query:
SELECT
post_id,
COUNT(
CASE
WHEN value = 1
THEN 1
END
) AS likes,
COUNT(
CASE
WHEN value = 0
THEN 1
END
) AS unlikes,
(CASE
WHEN user_id = 1 -- I choose user_id manually
THEN id
END) AS vote_id,
(CASE
WHEN user_id = 1
THEN value
END) AS user_vote
FROM votes
GROUP BY post_id;
What I expected was something like this:
+---------+-------+---------+---------+-----------+
| post_id | likes | unlikes | vote_id | user_vote |
+---------+-------+---------+---------+-----------+
| 103 | 1 | 0 | 1 | 1 |
| 105 | 2 | 1 | 7 | 1 |
| 106 | 1 | 0 | 3 | 1 |
| 108 | 0 | 2 | 4 | 0 |
+---------+-------+---------+---------+-----------+
But this is the result I have:
+---------+-------+---------+---------+-----------+
| post_id | likes | unlikes | vote_id | user_vote |
+---------+-------+---------+---------+-----------+
| 103 | 1 | 0 | 1 | 1 |
| 105 | 2 | 1 | NULL | NULL |
| 106 | 1 | 0 | 3 | 1 |
| 108 | 0 | 2 | 4 | 0 |
+---------+-------+---------+---------+-----------+
Those NULL values happen because of the last line of my query (GROUP BY post_id). If there's a duplicate post_id in table (for example, if some other user already voted for that post), CASE will ignore it and return NULL.
What should I do?

Answer 1

The problem with your query is the lack of aggregations on the last two columns. Assuming that a user can only vote once for a post, you are safe combining the values using MAX() (or MIN()) because there is at most one matching row:
SELECT v.post_id, SUM(value = 1) AS likes, SUM(value = 0) AS unlikes,
MAX(CASE WHEN user_id = 1 THEN vote_id END) AS user1_voteid,
MAX(CASE WHEN user_id = 1 THEN value END) AS user1_vote
FROM votes v
GROUP BY v.post_id;