Retrieving the TOP 10 data with arithmetic operation - mysql

Question

I want to retrieve the name of the user with the top 10 most voted post.
the formula in calculating the votes should be upvotes-downvotes. I also need to count the votes of that post.
Here's my
table user_info
id lname fname
1 abc1 abc1
2 abc2 abc2
3 abc3 abc3
4 abc4 abc4
5 abc5 abc5
6 abc6 abc6
7 abc7 abc7
table post
post_id post_message user_id
1 sup 1
2 good morning 2
3 hello 5
4 test 3
5 sample 4
table vote
vote_id user_id post_id vote_type
1 1 1 upvote
2 2 1 upvote
3 1 2 downvote
4 1 2 upvote
5 2 2 downvote
6 3 1 upvote
7 3 4 upvote
8 4 4 downvote
here's the query that I tried.
SELECT post_id, fname,lname,COUNT(CASE WHEN vote_type = 'upvote' then 1 ELSE NULL END) as "upvotes", COUNT(CASE WHEN vote_type = 'downvote' then 1 ELSE NULL END) as "downvotes"
FROM user_info
LEFT JOIN post ON
post.user_id = user_info.id
LEFT JOIN vote ON
post.user_id = user_info.id
ORDER BY 'upvotes'- 'downvotes' // is this possible?
LIMIT 10
My desired output is something like this.
post_id lname fname vote
1 abc1 abc1 3 // 3 upvotes
4 abc3 abc3 0 // 1upvote - 1 downvote
3 abc5 abc5 0 // no vote
5 abc4 abc4 0 // no vote
2 abc2 abc2 -1 // 1upvote - 2 downvotes
I don't know where I should put that column "vote" but it should be there in my query.
I created an sql fiddle for this

Answer 1

Fiddle
select p.post_id,u.fname,u.lname,
case when sum(upvotes)+sum(downvotes) is null then 0
else sum(upvotes)+sum(downvotes) end as vote
from post p left join
(
SELECT post_id,
CASE WHEN vote_type = 'upvote' then 1 ELSE 0 END as "upvotes",
CASE WHEN vote_type = 'downvote' then -1 ELSE 0 END as "downvotes"
FROM vote
) t
on p.post_id = t.post_id
left join user_info u on p.user_id = u.id
group by p.post_id
order by vote desc, sum(upvotes) desc, sum(downvotes) desc
limit 10;