MySQL: Cascade filtering by date intervals - mysql

Question

Can any help me with such type of query.
I have:
posts table
comments table
They are linked through comments.post_id = posts.post_id columns.
I user can filter post by comments for the past:
1 hour
24 hours
2 days, etc.
If user selected to show posts for the past 1 hour, but there no posts this period, we need to go step by step:
Select posts for past 1 hour, if empty - for past 24 hours, if empty - for past 2 days, if empty - since inception (without any conditions).
Could anyone please help me to build such query?
UPD
"Filter posts by comments" means sort by comments count.
So actually goal is request "Show me posts sorted by comments count that have been left for the past XXX hours".
And if is selected "for the past hour" but there are no posts with comments left for the past 1 hour, we need to fetch posts with comments left for the past 24 hours (sorted by comments count) and so on.
Tables structure
Posts:
post_id
title
content
date_added
Comments
comment_id
content
post_id
date_added
So link is posts.post_id = comments.post_id.
I would like to have next result when user view most commented posts for the past hour:
posts.post_id | comments_count | posts.date_added | group
---------------+----------------+------------------+----------------
156 | 8 | 2013-04-02 | hour
154 | 3 | 2013-04-02 | hour
129 | 1 | 2013-03-10 | 24 hours
13 | 14 | 2013-02-18 | 48 hours
138 | 6 | 2013-03-29 | week
137 | 4 | 2013-03-29 | week
161 | 21 | 2013-04-11 | month
6 | 2 | 2013-01-24 | year
103 | 8 | 2013-03-02 | since inception
Results sorted by:
Top of the list is 2 posts that have been commented due the past hour, and ordered by comments count.
Next we place posts that have been commented due the past day.
Next — posts commented due past 2 days
posts commented due past week, and again they should be ordered by comments count
For the past month
For the past year
In the end of this list we need to place articles that have been commented more than year ago, and they also should be ordered by comments count.
Thanks in advance.

Answer 1

Calculate the most recent comment for each group. Then use this to choose which group you want. You can do this calculation with a subquery:
select p.* c.*
from posts p join
comments c
on p.post_id = posts.post_id join
(select post_id, max(postdate) as postdate
from comments
group by post_id
) cmax
on cmax.post_id = p.post_id
where (case when timestampdiff(minute, now(), cmax.timestamp) <= 60
then timestampdiff(minute, now(), c.timestamp) <= 60
when timestampdiff(minute, now(), cmax.timestamp) <= 60*24
then timestampdiff(minute, now(), c.timestamp) <= 60*24
. . .
)
The syntax for the time comparison depends on whether the values are stored as timestamps or datetimes.

Answer 2

If you want the top 5 posts in the last hour, assuming your date_added fields are timestamps, you can use:
SELECT post_id,
count(comment_id) as comments_count,
posts.date_added, 'hour' as grouptime
FROM posts
INNER JOIN comments
ON posts.post_id = comments.post_id
WHERE TIMESTAMPDIFF(HOUR, comments.date_added, NOW()) = 0
GROUP BY posts.post_id
ORDER BY count(comment_id) DESC
LIMIT 5
If you want all of them, just remove LIMIT. For the last 24 hours:
SELECT post_id,
count(comment_id) as comments_count,
posts.date_added, '24 hours' as grouptime
FROM posts
INNER JOIN comments
ON posts.post_id = comments.post_id
WHERE TIMESTAMPDIFF(HOUR, comments.date_added, NOW()) < 24
GROUP BY posts.post_id
ORDER BY count(comment_id) DESC
LIMIT 5
and so on for different time periods.
If you want to get all of them in one go, use UNION between these queries.