从MySQL表中选择数据，其中来自另一个表的某些结果取决于条件（如果有）

Question

With a PHP code, I wish to order some MySQL results in a specific way.

I have two tables:

First table, called "post" with the following non-exhaustive content:

• id
• released_date
• published_date
• ...

Second table, called "votes" with the following non-exhaustive content:

• id
• post_id
• user_id
• status

If a user votes for an existing post, a record is made in the "votes" table, inserting the post id as post_id and the user id as user_id. The status is 1 for an upvote and 2 for a downvote.

One important thing to keep in mind though: some posts might never receive any vote at all, and 0 occurrence would be found in the second table, for instance!

So, now on my first page I order my posts with their amount of votes (upvotes-downvotes) the following way, and it works fine :

$post_req = mysql_query('SELECT * FROM post
WHERE NOW() > released_date
ORDER BY ((SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 1)-(SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 2)) DESC, published_date DESC
LIMIT 0, 5');

But on the next page (ie a new iteration), I need to continue where I left, in terms of votes. Before I do that, I save the votes amount of the last post of the first page to pass it to the next one, under the variable $last_post_votes.

Now, this where I struggle: I need to look for posts which have a lower amount of votes than this variable in a new request. As follows, my failing attempt:

$post_req_1 = mysql_query('SELECT * FROM post
WHERE NOW() > released_date
AND ((SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 1)-(SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 2)) < "'.$last_post_votes.'"
ORDER BY ((SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 1)-(SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 2)) DESC, published_date DESC
LIMIT 5');

And then I tried something like that, which failed too:

$post_req_1 = mysql_query('SELECT post.*,
((SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 1)-(SELECT COUNT(*) FROM votes WHERE votes.poste_id = post.id AND votes.status = 2)) AS all_votes
FROM post, votes
WHERE NOW() > post.released_date
AND all_votes < "'.$last_post_votes.'"
ORDER BY all_votes DESC, post.published_date DESC
LIMIT 5');

The problem is clearly the condition upon searching results from another table in the SELECT.

Any help is greatly appreciated!

Thanks a lot in advance! :-)

Lois

EDIT :

I managed to make it work as I wished, by using a condition directly in the ORDER BY itself. Not sure it's super proper, but it seems to work:

$post_req_1 = mysql_query('SELECT * FROM post
WHERE NOW() > released_date
ORDER BY ((SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 1)-(SELECT COUNT(*) FROM votes WHERE votes.post_id = post.id AND votes.status = 2)) < "'.$last_post_votes_passed.'" DESC, published_date DESC
LIMIT 5');

Answer 1

If your first statement works fine, why don't you continue with that?

Aou start with the LIMIT clause of LIMIT 0,5 which gives you the first 5 records of the resultset. If you increase the first parameter to LIMIt it will show you the next 5 records, and so on. No need to fiddle around with interim variables...

// first page
SELECT ... LIMIT 0,5

// seond page
SELECT ... LIMIT 5,5

// 10th page
SELECT ... LIMIT 45,5

See also http://dev.mysql.com/doc/refman/5.0/en/select.html

Answer 2

If i am not mistaken is this what you are looking for?

SELECT p.id, IFNULL(COUNT(v.id), 0) - IFNULL(COUNT(v2.id), 0) AS `Votes`
FROM
posts p 
LEFT JOIN votes v on v.post_id = p.id AND v.status = 1
LEFT JOIN votes v2 on v2.post_id = p.id and v.status = 2
WHERE NOW() > p.released_date
ORDER BY `Votes` DESC;