SQL ORDER BY something else than one of the table's columns - sql

Question

I have a table with posts in them. Website visitors can upvote or downvote such a post. I want to order a certain sql query by the score of the post, but my posts table doesn't have a score column - I keep the upvotes and downvotes in a different votes table, because that tells me who voted on what. I could add a score column to by posts table, and update it every time someone votes on a post, but I'd rather not do this, as the score is something I can work out by subtracting the downvotes from the upvotes anyways.
Do you have any suggestions? Or should I just go ahead and add a score column to my table?
Edit
My posts table has a post_id column (among other irrelevant columns) and my votes table has columns post_id, user_id and positive (the latter is a BOOLEAN, being 1 when the vote is an upvote and 0 when the vote is a downvote).
I can easily determine the score of a post 'by hand', by first querying the number of upvotes of that post, then the number of downvotes, and calculating their difference. However, I would like to query my posts table and order by the score of that post, so I want to know how/if I can query the votes table in the ORDER BY command while querying the posts table.

Answer 1

No, you do not have to create a score column. You can order by the calculated score, as below:
Since you do have the upvotes and downvotes in a different table, you need to join, as Tim Schmelter has explained.
SELECT p.*
FROM Post p
INNER JOIN Votes v
ON p.PostID = v.PostID
ORDER BY (v.upvotes - v.downvotes);
If you want to get the query to perform better, you could add a function-based index for (v.upvotes - v.downvotes).
EDIT:
Based on the updated information about the posts and the votes table, the following query can be used. The score is calculated within an inline view using a CASE statement. Then, this inline view is joined with the posts table, ordering the rows by the score. Note that an INNER JOIN is used, so only posts that have votes would be listed. To list all posts, a LEFT JOIN could be used instead.
SELECT p.*
FROM posts p
INNER JOIN
(
SELECT
post_id,
SUM
(
CASE
WHEN positive = 0 THEN -1
ELSE 1
END
) score
FROM votes v
GROUP BY post_id
) scores
ON p.post_id = scores.post_id
ORDER BY scores.score;

Answer 2

You have to link both tables via JOIN. Presuming that the Score-table has a column PostID:
SELECT p.*, Score = s.Upvotes- s.DownVotes
FROM Post p
INNER JOIN Score s
ON p.PostID = s.PostID
ORDER BY Score

Answer 3

Presumably, your data has a scores table with a column for each vote and an indicator of whether it is an up vote or down vote. If so, you need to aggregate this information and then you can use it for ordering:
select p.*, (NumUpVotes - NumDownVotes) as NetVotes
from posts p left outer join
(select PostId, sum(case when IsUpVote = 'Y' then 1 else 0 end) as NumUpvotes,
sum(case when IsDownVote = 'Y' then 1 else 0 end) as NumDownVotes
from scores s
group by PostId
) s
on p.postId = scores.PostId
order by (NumUpVotes - NumDownVotes);
You don't specify what database you are using so this uses standard SQL that should work in any database. You can adapt the logic for your particular data structure.