In the application I'm building, there are posts and tags, and they are connected through a many-to-many relation. What I want to do is show all tags to the user and sort them by how many published posts they have (which is determined by the is_published column in the posts table).
Right now I'm sorting them by how many posts they have in general (both published and unpublished) with this code:
scope :top_used, -> { left_joins(:posts).group(:id).order("COUNT(posts.id) DESC") }
Which translates to the MySQL:
SELECT t.*
FROM tags t
LEFT
JOIN post_tags pt
ON pt.tag_id = tags.id
LEFT
JOIN posts p
ON p.id = pt.post_id
GROUP
BY t.id
ORDER
BY COUNT(p.id) DESC
So, again, I need to sort by the count of the number of published posts not all posts. Can this be done in MySQL?
Maybe this:
SELECT `tags`.* FROM `tags`
LEFT OUTER JOIN `post_tags` ON `post_tags`.`tag_id` = `tags`.`id`
LEFT OUTER JOIN `posts` ON `posts`.`id` = `post_tags`.`post_id` AND posts.is_published = 1
GROUP BY `tags`.`id`
ORDER BY COUNT(posts.id) DESC
?
Assuming that the column is_published's data type is Boolean or Integer, with values 1 or 0 you can order by the sum of the values:
ORDER BY SUM(posts.is_published) DESC
If is_published is nullable, use COALESCE():
ORDER BY COALESCE(SUM(posts.is_published), 0) DESC
Related
I'm currently creating a small application where users can post a text which can be commented and the post can also be voted (+1 or -1).
This is my database:
Now I want to select all information of all posts with status = 1 plus two extra columns: One column containing the count of comments and one column containing the sum (I call it score) of all votes.
I currently use the following query, which correctly adds the count of the comments:
SELECT *, COUNT(comments.fk_commented_post) as comments
FROM posts
LEFT JOIN comments
ON posts.id_post = comments.fk_commented_post
AND comments.status = 1
WHERE posts.status = 1
GROUP BY posts.id_post
Then I tried to additionally add the sum of the votes, using the following query:
SELECT *, COUNT(comments.fk_commented_post) as comments, SUM(votes_posts.type) as score
FROM posts
LEFT JOIN comments
ON posts.id_post = comments.fk_commented_post
AND comments.status = 1
LEFT JOIN votes_posts
ON posts.id_post = votes_posts.fk_voted_post
WHERE posts.status = 1
GROUP BY posts.id_post
The result is no longer correct for either the votes or the comments. Somehow some of the values seem to be getting multiplied...
This is probably simpler using correlated subqueries:
select p.*,
(select count(*)
from comments c
where c.fk_commented_post = p.id_post and c.status = 1
) as num_comments,
(select sum(vp.type)
from votes_posts vp
where c.fk_voted_post = p.id_post
) as num_score
from posts p
where p.status = 1;
The problem with join is that the counts get messed up because the two other tables are not related to each tother -- so you get a Cartesian product.
You want to join comments counts and votes counts to the posts. So, aggregate to get the counts, then join.
select
p.*,
coalesce(c.cnt, 0) as comments,
coalesce(v.cnt, 0) as votes
from posts p
left join
(
select fk_commented_post as id_post, count(*) as cnt
from comments
where status = 1
group by fk_commented_post
) c on c.id_post = p.id_post
left join
(
select fk_voted_post as id_post, count(*) as cnt
from votes_posts
group by fk_voted_post
) v on v.id_post = p.id_post
where p.status = 1
order by p.id_post;
SQL Query:
SELECT
T.*,
U.nick AS author_nick,
P.id AS post_id,
P.name AS post_name,
P.author AS post_author_id,
P.date AS post_date,
U2.nick AS post_author
FROM
zero_topics T
LEFT JOIN
zero_posts P
ON
T.id = P.topic_id
LEFT JOIN
zero_players U
ON
T.author = U.uuid
LEFT JOIN
zero_players U2
ON
P.author = U2.uuid
ORDER BY
CASE
WHEN P.date is null THEN T.date
ELSE P.date
END DESC
Output:
Topics:
Posts:
Question: Why i have duplicated topic id 22? i have in mysql two topics (id 22 and 23) and two posts(id 24 and 25). I want to see topic with last post only.
If a join produces multiple results and you want only at most one result, you have to rewrite the join and/or filtering criteria to provide that result. If you want only the latest result of all the results, it's doable and reasonably easy once you use it a few times.
select a.Data, b.Data
from Table1 a
left join Table2 b
on b.JoinValue = a.JoinValue
and b.DateField =(
select Max( DateField )
from Table2
where JoinValue = b.JoinValue );
The correlated subquery pulls out the one date that is the highest (most recent) value of all the joinable candidates. That then becomes the row that takes part in the join -- or, of course, nothing if there are no candidates at all. This is a pattern I use quite a lot.
I'm a little bit confused about a stupid query:
I get rows from the table posts joined with the table authors and the table comments, in a way like this:
SELECT posts.*, authors.name, COUNT(comments.id_post) AS num_comments
FROM posts JOIN authors ON posts.id_author = authors.id_author
LEFT JOIN comments ON posts.id_post = comments.id_post
WHERE posts.active = 1
AND comments.active = 1
this doesn't work, of course.
What I try to do is to retrieve:
1) all my active post (those that were not marked as deleted);
2) the names of their authors;
3) the number of active comments (those that were not marked as deleted) for each post (if there is at least one);
What's the way? I know it's a trivial one, but by now my brain is in offside…
Thanks!
Presumably, id_post uniquely identifies each row in posts. Try this:
SELECT p.*, a.name, COUNT(c.id_post) AS num_comments
FROM posts p JOIN
authors a
ON p.id_author = a.id_author LEFT JOIN
comments c
ON p.id_post = c.id_post
WHERE p.active = 1 AND c.active = 1
GROUP BY p.id_post;
Note that this uses a MySQL extension. In most other databases, you would need to list all the columns in posts plus a.name in the group by clause.
EDIT:
The above is based on your query. If you want all active posts with a count of active comments, just do:
SELECT p.*, a.name, SUM(c.active = 1) AS num_comments
FROM posts p LEFT JOIN
authors a
ON p.id_author = a.id_author LEFT JOIN
comments c
ON p.id_post = c.id_post
WHERE p.active = 1
GROUP BY p.id_post;
Since you are doing a count, you need to have a group by. So you will need to add
Group By posts.*, authors.name
You should you GROUP BY clause together with aggregate functions. Try something similar to:
SELECT posts.*, authors.name, COUNT(comments.id_post) AS num_comments
FROM posts JOIN authors ON posts.id_author = authors.id_author
LEFT JOIN comments ON posts.id_post = comments.id_post
-- group by
GROUP BY posts.*, authors.name
--
WHERE posts.active = 1
AND comments.active = 1
I found the correct solution:
SELECT posts.id_post, authors.name, COUNT(comments.id_post) AS num_comments
FROM posts JOIN authors
ON posts.id_author = authors.id_author
LEFT OUTER JOIN comments
ON (posts.id_post = comments.id_post AND comments.active = 1)
WHERE posts.active = 1
GROUP BY posts.id_post;
Thanks everyone for the help!
Could someone hint me in the direction of how to merge these two mysql queries?
As you see there is a bug with the distinct too. I need the topicids without duplicates and the date for the post from which the topicid comes.
I added the distinct just to get you an idea of what i need. After getting the ids there comes the second query which reads the topicdetails...
I am stuck on this.
Get the unique topicid from the last n,15 posts for a category, but with the date:
SELECT distinct( p.topicid ), p.date as lastaction
FROM posts p
WHERE p.category = ?
ORDER BY p.id
DESC
LIMIT
n, 15
SELECT t.id, t.title, t.date, t.state
FROM topics t
WHERE t.id in( $var_ids )
thanks.
SELECT DISTINCT
p.[topicid],
p.[date] AS lastaction,
t.[id],
t.[title],
t.[date],
t.[state]
FROM posts p
INNER JOIN topics t
ON p.topicid = t.id
WHERE p.category = /* Enter Category */ AND t.id in ( $var_ids )
ORDER BY p.topicid DESC
Use this as a starter for 10. I'm unsure what 'n' is (e.g Column?) so i've left this out
Basically I want to select all the posts that have at least one comment and get the comment count for each. What I cam up with so far is only giving me one result, which is has an accurate count but there are more than one posts that have comments. Can any recommend changes to make this work
SELECT
posts.id,
COUNT(DISTINCT comments.post_id) AS count
FROM
posts
LEFT JOIN comments ON posts.id = comments.post_id
Using aggregate functions you must group them see here GROUP BY (Aggregate) Functions for the posts must contain atleast one comment you can use HAVING count >= 1
SELECT
posts.id,
COUNT(DISTINCT comments.post_id) AS `count`
FROM
posts
LEFT JOIN comments ON posts.id = comments.post_id
GROUP BY posts.id
HAVING `count` >= 1
You need a group by statement. And you can change the join to an inner join because you want only posts with comments:
SELECT p.id, COUNT(*) AS count
FROM posts p INNER JOIN
comments c
ON p.id = c.post_id
GROUP BY p.id;
The expression count(distinct c.post_id) will return 1 for each row, because there is only one distinct post id for each row. COUNT(*) will get the number of comments.