Mysql select unique field from a groupped table - mysql

Question

I have table named as posts
|id|user_id|title|description|
likes table like this
|id|user_id|post_id|
here is my sql code
select count(likes.id) as like_count, posts.* from `posts` left join
`likes` on `likes`.`post_id` = `posts`.`id` where `posts`.`status` = 'A'
group by `posts`.`id` order by `like_count` desc LIMIT 6
Here is the result
|like_count | id | user_id | title | description |
---------------------------------------------------
17 | 723 | 12 | ... | .... |
15 | 721 | 15 | ... | .... |
14 | 711 | 12 | ... | .... |
13 | 700 | 12 | ... | .... |
12 | 800 | 11 | ... | .... |
12 | 920 | 12 | ... | .... |
as you see user_id 12 is repeating thee times, I want only one post from one user.
Is there a way to select most liked post of each user, and make user_id don't repeat.

Answer 1

I'm aware this may not be the most efficient solution, but this is the first that came to my mind. Unfortunately, MySQL doesn't support window functions so the task is way harder and less elegant than the approach one would take using window functions.
Most liked posts of each user which will repeat user_id if there happens to be more than one post with equal number of likes and that number is the greatest for particular user:
select
user_likes.post_user_id,
user_likes.like_count,
p.id as post_id,
p.title,
p.description
from (
select
p.user_id as post_user_id,
MAX(l.like_count) as like_count
from posts p
inner join (select post_id, count(*) as like_count from likes group by post_id) l on p.id = l.post_id
group by p.user_id
) user_likes
inner join (select post_id, count(*) as like_count from likes group by post_id) l on user_likes.like_count = l.like_count
inner join posts p on l.post_id = p.id
If you really need one row per user then in case of ties in number of likes for different posts you need to pick a strategy to retrieve only one of those. For example you could take the one that was older by adding grouping on post_id and taking the min() of its value and joining back to posts table to retrieve it's title and description.
In this case modification of above script would look like:
select
most_liked_post.post_user_id,
most_liked_post.like_count,
most_liked_post.post_id,
p.title,
p.description
from (
select
user_likes.post_user_id,
user_likes.like_count,
MIN(l.post_id) AS post_id
from (
select
p.user_id as post_user_id,
MAX(l.like_count) as like_count
from posts p
inner join (select post_id, count(*) as like_count from likes group by post_id) l on p.id = l.post_id
group by p.user_id
) user_likes
inner join (select post_id, count(*) as like_count from likes group by post_id) l on user_likes.like_count = l.like_count
group by user_likes.post_user_id, user_likes.like_count
) most_liked_post
inner join posts p on most_liked_post.post_id = p.id