I have an SQLite table blog_posts. Every blog post has an id and blog_id.
If I want to know how many blog posts every blog has:
SELECT blog_id, count(1) posts FROM blog_posts group by blog_id
What do I do if I want to know how many posts the blog with the most posts has? (I don't need the blog_id.) Apparently this is illegal:
SELECT max(count(1)) posts FROM blog_posts group by blog_id
I'm pretty sure I'm missing something, but I don't see it...
Other solution:
select count(*) as Result from blog_posts
group by blog_id
order by Result desc
limit 1
I'm not sure which solution would run faster, if this one or the one with the subquery.
You can use a subquery. Here's how you do it:
get the number of posts for each blog
select the maximum number of posts
Example:
select max(num_posts) as max_posts
from (
select blog_id, count(*) as num_posts
from blog_posts
group by blog_id
) a
(The subquery is in the (...)).
NB: I'm not a SQLite power user and so I don't know if this works, but the SQLite docs indicate that subqueries are supported.
Related
Okay, so i have a list of posts and some posts are replies to other posts. I'd like to get a list of post parents in reverse order of replies.
I've tried group by but it always lists the wrong order and distinct is the only way i've managed to get it to work but obviously then it only lists the post id and not the rest of the data.
example of database here
The order i want to pull the posts out in is 1,3,5,4,2 These are the non-reply posts in the order of the latest reply.
SELECT DISTINCT `thread`
FROM
(
SELECT COALESCE(NULLIF(`parent_post`, 0), `postID`) AS `thread`
FROM `posts`
ORDER BY `postID` DESC
LIMIT 100
) `sub`
This pulls them out in the correct order but obviously only pulls out the postID and not the rest of the fields, i've tried group by but it loses the correct order.
A straightforward translation of your requirements to SQL would be:
select *
from posts p1
where parent_post = 0
order by (
select max("datetime")
from posts p2
where p2.parent_post = p1.postID
) desc
I.e. select all rows from posts that are thread starters (not replies) and order them by the latest timestamp from any of their replies in descending order.
Having the classic "posts table, and comments table with foreign key to posts table" scenario, what's the most efficient way to get the IDs of the last 20 posts ordered by the time of their last comment, and the actual comment itself?
Here is a query that works but can probably be done much more efficiently:
SELECT * FROM (
SELECT * FROM comments ORDER BY time DESC
) AS foo GROUP BY post_id ORDER BY time DESC LIMIT 20
A nested query with an ORDER BY is necessary to make sure that the latest comment gets selected into the post_id group.
As mentioned in the comments: practically the same question as Retrieveing the most recent records within a query.
See the greatest-n-per-group tag for more similar questions.
I have 2 tables. One table contains posts and the other contains votes for the posts. Each member can vote (+ or -) for each post.
(Structure example:)
Posts table: pid, belongs, userp, text.
Votes table: vid, userv, postid, vote.
Also one table which contains the info for the users.
What I want is: Supposing I am a logged-in member. I want to show all the posts, and at those I've already voted, not let me vote again. (and show me what I have voted + or -)
What I have done til now is very bad as it will do a lot of queries:
SELECT `posts`.*, `users`.`username`
FROM `posts`,`users`
WHERE `posts`.belongs=$taken_from_url AND `users`.`usernumber`=`posts`.`userp`
ORDER BY `posts`.`pid` DESC;
and then:
foreach ($query as $result) {if (logged_in) {select vote from votes....etc} }
So, this means that if I am logged in and it shows 30 posts, then it will do 30 queries to check if at each post I have voted and what I've voted. My question is, can I do it shorter with a JOIN (I guess) and how? (I already tried something, but didn't succeed)
Firstly I'll say that if you're going to have significantly different output for users logged in versus those that aren't, just have two queries rather than trying to create something really complicated.
Secondly, this should do something like what you want:
SELECT p.*, u.username,
(SELECT SUM(vote) FROM votes WHERE postid = p.pid) total_votes,
(SELECT vote FROM votes WHERE postid = p.pid AND userv = $logged_in_user_id) my_vote
FROM posts p
JOIN users u ON p.userp = u.usernumber
WHERE p.belongs = $taken_from_url
ORDER BY p.pid DESC
Note: You don't say what the values of the votes table are. I'm assuming it's either +1 (up) or -1 (down) so you can easily find the total votes by adding them up. If you're not doing it this way I suggest you do to make your life easier.
The first correlated subquery can be eliminated by doing a JOIN and GROUP BY but I tend to find the above form much more readable.
So what this does is it joins users to posts, much like you were doing except that it uses JOIN syntax (which again comes down to readability). Then it has two subqueries: the first finds the total votes for that particular post and the second finds out what a particular user's vote was:
+1: up vote;
-1: down vote;
NULL: no vote.
So I'm trying to build a simple forum. It'll be a list of topics in descending order by the date of either the topic (if no replies) or latest reply. Here's the DB structure:
Topics
id, subject, date, poster
Posts
id, topic_id, message, date, poster
The forum itself will consist of an HTML table with the following headers:
Topic | Last Post | Replies
What would the query or queries look like to produce such a structure? I was thinking it would involve a cross join, but not sure... Thanks in advance.
Of course you can make a query for this, but I advise you to create in Topics table fields 'replies' and 'last post', then update them on every new post. That could really improve your database speed, not now, but the time when you will have thousands of topics.
SELECT *
FROM
`Topics`,
(
SELECT *, COUNT(*) AS `replies`
FROM `Posts`
GROUP BY `Posts`.`topic_id`
ORDER BY `Posts`.`date` DESC
) AS `TopicPosts`
WHERE `Topics`.`id` = `TopicPosts`.`topic_id`
ORDER BY `Posts`.`date` DESC
This 'should' work, or almost work in the case it doesn't, but I agree with the other poster, it's probably better to store this data in the topics table for all sorts of reasons, even if it is duplication of data.
The forum itself will consist of an
HTML table with the following headers:
Topic | Last Post | Replies
If "Last Post" is meant to be a date, it's simple.
SELECT
t.id,
t.subject,
MAX(p.date) AS last_post,
COUNT(p.id) AS count_replies
FROM
Topics t
INNER JOIN Posts p ON p.topic_id = t.id
GROUP BY
t.id,
t.subject
If you want other things to display along with the last post date, like its id or the poster, it gets a little more complex.
SELECT
t.id,
t.subject,
aggregated.reply_count,
aggregated.distinct_posters,
last_post.id,
last_post.date,
last_post.poster
FROM
Topics t
INNER JOIN (
SELECT topic_id,
MAX(p.date) AS last_date,
COUNT(p.id) AS reply_count,
COUNT(DISTINCT poster) AS distinct_posters
FROM Posts
GROUP BY topic_id
) AS aggregated ON aggregated.topic_id = t.id
INNER JOIN Posts AS last_post ON p.date = aggregated.last_date
As an example, I've added the count of distinct posters for a topic to show you where this approach can be extended.
The query relies on the assumption that no two posts within one topic can ever have the same date. If you expect this to happen, the query must be changed to account for it.
I have asked a similar question before and while the answers I got were spectacular I might need to clearify.
Just like This question I want to return N number of rows depending on a value in a column.
My example will be I have a blog where I want to show my posts along with a preview of the comments. The last three comments to be exact.
I have have I need for my posts but I am racking my brain to get the comments right. The comments table has a foreign key of post_id which obviously multiple comments can be attached to one post so if a post has 20 comments then I just want to return the last three. What makes this somewhat tricky is I want to do it in one query and not a "limit 3" query per blog post which makes rendering a page with a lot of posts very query heavy.
SELECT *
FROM replies
GROUP BY post_id
HAVING COUNT( post_id ) <=3
This query does what I want but only returns one of each comment and not three.
SELECT l.*
FROM (
SELECT post_id,
COALESCE(
(
SELECT id
FROM replies li
WHERE li.post_id = dlo.post_id
ORDER BY
li.post_id, li.id
LIMIT 2, 1
), CAST(0xFFFFFFFF AS DECIMAL)) AS mid
FROM (
SELECT DISTINCT post_id
FROM replies dl
) dlo
) lo, replies l
WHERE l.replies >= lo.replies
AND l.replies <= lo.replies
AND l.id <= lo.mid
Having an index on replies (post_id, id) (in this order) will greatly improve this query.
Note the usage of l.replies >= lo.replies AND l.replies <= lo.replies: this is to make the index to be usable.
See the article in my blog for details:
Advanced row sampling (how to select N rows from a table for each GROUP)
Do you track comment date? You can sort those results to grab only the 3 most recent ones.
following ian Jacobs idea
declare #PostID int
select top 3 post_id, comment
from replies
where post_id=#PostID
order by createdate desc