What I want:
SELECT u.username, u.last_activity
FROM users_userprofile
WHERE u.id IN (
SELECT DISTINCT(p.user_id) FROM forums_post p
WHERE p.thread_id = 423993
ORDER BY p.created_at DESC
LIMIT 4
);
This doesn't work because of LIMIT in subquery. I want to keep order of subquery but I want to get username and last_activity instead of user_id.
Any suggestion how I could achieve this?
Replace the subquery with a view:
CREATE VIEW subv AS SELECT p.user_id FROM forums_post p
WHERE p.thread_id = 423993
ORDER BY p.created_at DESC
LIMIT 4;
SELECT u.username, u.last_activity
FROM users_userprofile
WHERE u.id IN (SELECT * FROM subv);
you could use join for the table and the subquery instead of using where in:
SELECT u.username, u.last_activity
FROM users_userprofile u
JOIN (
SELECT p.user_id FROM forums_post p
WHERE p.thread_id = 423993
ORDER BY p.created_at DESC
LIMIT 4
) q
on u.user_id=q.user_id
Why woldn't you do it with a JOIN? There seems to be no performance impact because WHERE and LIMIT clauses are the same. It won't JOIN the whole tables:
SELECT p.user_id, u.username, u.last_activity
FROM users_userprofile u
JOIN forums_post p ON p.user_id = u.id
WHERE p.thread_id = 423993
GROUP BY p.user_id ORDER BY MAX(p.created_at) DESC
LIMIT 4
Related
Consider the following query:
SELECT u.id
, u.name
, p.id
FROM users u
LEFT
JOIN posts p
ON p.id IN(
SELECT x.id
FROM posts x
WHERE x.user_id = u.id
ORDER
BY x.featured DESC
LIMIT 10
);
I am trying to join the posts table to the users table. However I only want to retrieve a maximum of 10 posts per user.
This approach throws the following error:
This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
Is there an alternative approach to achieve the desired result?
You should use an inner join on subquery instead of left join and IN clause
SELECT users.id, users.name, t.id
FROM users
INNER JOIN (
SELECT posts.id, posts.user_id
FROM posts
WHERE posts.user_id = users.id
ORDER BY posts.featured DESC
LIMIT 10 ) t on t.user_id = users.id
In this way you don't use a limit inside a In clause for subquery and this should not raise the error .. the error for both LIMIT & IN .... but not for athe sue of in limit in subquery in JOIN (withou IN clause)
I'm trying to retrieve the last 10 posts from a posts table ordered ASC, but the last left joined query doesn't retrieve anything.
Basic: it retrieves results ordered DESC
SELECT
p.post, p.id_post, u.name
FROM
posts p
LEFT JOIN
users u ON u.id_user = p.id_user
WHERE
p.id_user = 4
ORDER BY
p.date DESC
LIMIT 10
Ordered ASC: it doesn't work at all:
SELECT
num.*
FROM
(SELECT
p.post, p.id_post, u.name
FROM
posts p
LEFT JOIN
users u ON u.id_user = p.id_user
WHERE
p.id_user = 4
ORDER BY
p.date DESC
LIMIT 10) num
ORDER BY
p.date ASC
What am I doing wrong?
In my php variables I use $row['id_post'] $row['post'] $row['name']. I don't want to use array_reverse() just plain sql
You need to return the date in the subquery:
SELECT pu.post, pu.id_post, pu.name
FROM (SELECT p.*, u.name
FROM posts p LEFT JOIN
users u
ON u.id_user = p.id_user
WHERE p.id_user = 4
ORDER BY p.date DESC
LIMIT 10
) pu
ORDER BY pu.date ASC;
Your second query should have returned an error message to the effect that date is not recognized as a column. You should be capturing and reading error messages, if you want to write an effective application.
I have this query:
SELECT * FROM project as p inner join users as u on p.user_id = u.user_id
ORDER BY p.projectname, u.date_joined
Now what I want to ORDER by projectname ASC and u.date_joined DESC.
How could I make it this way? I tried this code here but doesn't work:
SELECT * FROM project as p inner join users as u on p.user_id = u.user_id
ORDER BY p.projectname ASC, u.date_joined DESC
Order the projectname ascending works but the descending won't work.
To order project name descending, do this:
SELECT *
FROM project p
INNER JOIN users u ON p.user_id = u.user_id
ORDER BY p.projectname DESC, u.date_joined DESC
This will order your result by projectname desc (Z to A) first. For identical projectname records, date_joined will be used for sorting further (latest date first).
I am looking to load all posts, sorted by newest first, but also limit them to three per user. I have no idea how to do that though! Here's the SQL I have currently to create the table and select the posts.
SELECT p.title, u.firstname, u.lastname
FROM post p
JOIN user u
ON p.user_id=u.id
ORDER
BY u.id, p.ctime DESC
#LIMIT TO 3 by user
;
This is what you can do
select
u.*,
p.*
from
user u
left join
(
select
p1.*
FROM
post p1
where
(
select
count(*)
from
post p2
WHERE
p1.user_id = p2.user_id
AND p1.id <= p2.id
) <= 3
order by p1.id desc
) p ON u.id = p.user_id
order by u.id
Took help from MySQL Limit LEFT JOIN Subquery after joining
Something like this might work
SELECT u.*,up.* FROM user u LEFT JOIN
(
SELECT `p`.`title`, `u`.`firstname`, `u`.`lastname`
FROM `post` `p`
ORDER
BY `p`.`ctime` DESC
LIMIT 3
)
up on up.user_id = u.id
i have this
SELECT COUNT(1) cnt, a.auther_id
FROM `posts` a
LEFT JOIN users u ON a.auther_id = u.id
GROUP BY a.auther_id
ORDER BY cnt DESC
LIMIT 20
its work fine
bu now i want select from posts which added from 1 day tried to use
WHERE from_unixtime(post_time) >= SUBDATE(NOW(),1)
but its didnot worked
any one have idea
My guess is that you added the WHERE clause in the wrong place. It should come after the JOIN but before the GROUP BY, like this:
SELECT COUNT(1) cnt, a.auther_id
FROM `posts` a
LEFT JOIN users u ON a.auther_id = u.id
WHERE from_unixtime(post_time) >= SUBDATE(NOW(),1)
GROUP BY a.auther_id
ORDER BY cnt DESC
LIMIT 20