Selecting the last record and comparing a datetime - mysql

Question

I'm building a discussion board and I want to get a list of unread topics.
A topic should be unread and selected if the created_at datetime for the last post in a topic is greater than the last time the currently logged in user viewed this topic.
http://sqlfiddle.com/#!2/4e2e99/1
If you delete all of the user view inserts ALL of the topics should be listed.
I have three tables:
topics
id
user_id
created_at
topic_posts
id
topic_id
created_at
topic_user_views
id
topic_id
created_at
My query so far (but it doesn't work):
SELECT DISTINCT `topics`.`id`, `topics`.`name`
FROM `topics`
INNER JOIN `topic_user_views`
ON `topic_user_views`.`topic_id` = `topics`.`id`
INNER JOIN `topic_posts`
ON `topic_posts`.`topic_id` = `topics`.`id`
WHERE `topic_posts`.`created_at` > `topic_user_views`.`created_at`
AND `topic_user_views`.`user_id` = 1
ORDER BY `id` DESC
I don't know how to compare a topic's last post's created_at column post to the last time this user has viewed the topic.

Answer 1

here is one way of doing it. we will use exists to test if the topic was viewed before the last topic post was created. see the sql fiddle - http://sqlfiddle.com/#!2/4e2e99/11
select t.id as topic_id, t.name as topic_name
from topics t
where not exists(
select tuv.topic_id, max(tuv.created_at) as last_view, max(tp.created_at) as last_post
from topic_user_views tuv
inner join topic_posts as tp
on tuv.topic_id=tp.topic_id and tuv.created_at > tp.created_at
group by topic_id
having t.id=topic_id)
ORDER BY id DESC

Answer 2

With this example, topics also will be selected if user have not seen anything yet:
SELECT DISTINCT
`topics`.`id`,
`topics`.`name`
FROM `topics`
LEFT JOIN `topic_user_views` AS tuv
ON `tuv`.`topic_id` = `topics`.`id`
WHERE (SELECT
1
FROM topic_posts AS tp
WHERE tp.topic_id = `topics`.`id`
AND (tp.created_at > tuv.created_at
OR tuv.created_at IS NULL)
LIMIT 1)IS NOT NULL
ORDER BY `id` DESC;

Answer 3

Here is what I came up with :
SELECT t1.topic_id, t3.account_user_id, t1.created_at last_post_date, t4.created_at as last_seen_date
FROM topic_posts t1
INNER JOIN (SELECT topic_id, MAX(created_at) as created_at
FROM topic_posts
GROUP BY topic_id) t2
USING (topic_id, created_at)
INNER JOIN topic_user_views t3
USING (topic_id)
INNER JOIN (SELECT topic_id, account_user_id, created_at
FROM topic_user_views
INNER JOIN (SELECT topic_id, account_user_id, MAX(created_at) as created_at
FROM topic_user_views
GROUP BY topic_id, account_user_id) _
USING (topic_id, created_at, account_user_id)) t4
ON t1.topic_id = t4.topic_id and t3.account_user_id = t4.account_user_id and t3.created_at = t4.created_at
WHERE t1.created_at > t4.created_at
AND t3.account_user_id = 1;
This idea is to join the date of the last posted message in a topic (t2) and the date of the last seen message in a topic by a user (t4), and then you just have to filter out the results you don't want.
If I'm not mistaken, in the SQLFiddle you provided, there are no topics that have not already been seen by users, so this returns nothing. I slightly modified it (you can see it here) and it seems to work as wanted.
To consider the topics never viewed by a user, I think the best solution is to use the account table with a RIGHT OUTER JOIN. Something similar to :
SELECT t1.topic_id, t3.account_user_id, t1.created_at last_post_date, t4.created_at as last_seen_date
FROM topic_posts t1
INNER JOIN (SELECT topic_id, MAX(created_at) as created_at
FROM topic_posts
GROUP BY topic_id) t2
USING (topic_id, created_at)
INNER JOIN topic_user_views t3
USING (topic_id)
INNER JOIN (SELECT topic_id, account_user_id, created_at
FROM topic_user_views
INNER JOIN (SELECT topic_id, account_user_id, MAX(created_at) as created_at
FROM topic_user_views
GROUP BY topic_id, account_user_id) _
USING (topic_id, created_at, account_user_id)) t4
ON t1.topic_id = t4.topic_id and t3.account_user_id = t4.account_user_id and t3.created_at = t4.created_at
RIGHT OUTER JOIN account
ON account.user_id = t3.account_user_id
WHERE t1.created_at > t4.created_at or t4.account_user_id IS NULL
AND t3.account_user_id = 1;
which I did not test.