retrieve another users posts shared by followeds sql, mysql - mysql

Question

I have this arrangement:
I want to retrieve all the posts shared by followeds, bearing in mind those shared posts doesnt belong to another followeds or the main follower.
TABLE USERS
id_user | name
15 Joe
16 Dan
17 Tom
TABLE COMMUNITY
id_follower | followed
15 16
15 17
TABLE POSTS
id_post | id_user | post
1 15 hi1
2 16 hi2
3 15 hi3
4 17 hi4
5 15 hi5
6 16 hi6
7 17 hi7
8 18 hi8
9 14 hi9
I'm adding up a new table "shares".
TABLE SHARES
id_sharer | id_post
16 4
14 8
17 8
16 9
EXPECTED OUTPUT POSTS
17 8 hi7 (shared by Tom)
16 9 hi6 (shared by Dan)
How shouldit be the sql query to get this new arrangement?
I was thinking adding a line LEFT JOIN shares l ON l.id_sharer=p.id_user
SELECT p.*,
FROM posts p
LEFT JOIN users u ON u.id_user=c.id_followed
LEFT JOIN shares l ON l.id_sharer=c.id_followed
LEFT JOIN community c ON c.id_follower=p.id_user
WHERE p.id_user=15
ORDER BY p.date_post DESC
*Tested but not retrieve as expected

Answer 1

Here is a first crack at this for you:
SELECT p.id_post, p.id_user, p.post, IF(l.date_like IS NOT NULL, l.date_like, p.date_post) AS date, l.id_liker
FROM posts p
LEFT JOIN likes l ON l.id_post = p.id_post
LEFT JOIN users u ON u.id_user = l.id_liker
ORDER BY l.date_like DESC
GROUP BY p.id_post
As a quick walk through of how I am approaching the problem, first I am LEFT JOINing the likes and ordering them in descending order, just as you did. Then I am grouping on the post id in order to ensure I only get one line per post. Then in the selector I am checking to see if the like date is null and substituting in the date of the post instead of the most recent like if there are no likes.
UPDATE
This should add the logic to ensure that only the likes of followers will count. Since one probably cannot follow themselves this should also keep one's own likes from affecting position.
SELECT p.id_post, p.id_user, p.post, IF(l.date_like IS NOT NULL, l.date_like, p.date_post) AS date, l.id_liker
FROM posts p
LEFT JOIN likes l ON l.id_post = p.id_post
LEFT JOIN users u ON u.id_user = l.id_liker
LEFT JOIN community c ON l.id_liker = c.followed
WHERE c.id_follower = 15
ORDER BY l.date_like DESC, p.date_post DESC
GROUP BY p.id_post

Answer 2

I understand that 'TABLE POSTS'.'id_user' is the author of the post and matches to 'TABLE USERS'.'id_user' and that you want to see the Posts that have Entries in 'TABLE LIKES' first and then all the others.
So I suggest to the following JOIN and ORDER BY
select p.*, l.date_like, u.* from posts p
left join likes l on p.id_post=l.id_post
left join users u on l.id_liker=u.id_user
order by l.date_like desc, p.date_post desc