Multiple left joins show distinct result with last date - mysql

Question

I want to build a discussion board. Therefore I have 3 tables: t_posts, t_boards, t_board_categories. Posts contain Threads (where post_post_id IS NULL) and replies to threads (where post_post_id IS NOT NULL), so it is self-referring. Now I want to show the last 5 posts (Threads and/or Replies) in all boards of all categories. The sql currently looks like this:
SELECT p.post_id,
p.post_board_id,
p.post_user_id,
p2.post_title,
bc.bc_name,
Max(p.post_date) post_date
FROM t_posts p
LEFT JOIN t_posts p2
ON ( p2.post_post_id IS NULL
AND ( p2.post_id = p.post_id
OR p.post_post_id = p2.post_id ) )
LEFT JOIN t_boards b
ON b.board_id = p.post_board_id
LEFT JOIN t_board_categories bc
ON bc.bc_id = b.board_bc_id
WHERE p.post_system_id = '1'
AND p.post_deleted = 0
AND bc.bc_deleted = 0
AND b.board_deleted = 0
GROUP BY p.post_id,
p2.post_id
ORDER BY p2.post_date DESC
LIMIT 5
The result is almost what I want, except of, that multiple replies in one thread will lead to showing only that one thread. What I want is 5 different threads to be shown with the most recent answers, not only one thread 5 times.
I join t_posts p2 for showing the original thread title.
I also tried adding DISTINCT p2.post_id to the SELECT, but it does not change anything.
Schema:
CREATE TABLE `t_board_categories` (
`bc_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`bc_name` varchar(128) COLLATE utf8_unicode_ci DEFAULT '',
`bc_prio` int(11) NOT NULL DEFAULT '0',
`bc_system_id` int(10) unsigned NOT NULL DEFAULT '0',
`bc_deleted` tinyint(1) NOT NULL DEFAULT '0',
PRIMARY KEY (`bc_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
CREATE TABLE `t_boards` (
`board_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`board_bc_id` int(10) unsigned DEFAULT NULL,
`board_name` varchar(128) COLLATE utf8_unicode_ci DEFAULT NULL,
`board_subtitle` text COLLATE utf8_unicode_ci,
`board_prio` int(11) NOT NULL DEFAULT '0',
`board_system_id` int(10) unsigned NOT NULL DEFAULT '0',
`board_deleted` tinyint(4) NOT NULL DEFAULT '0',
PRIMARY KEY (`board_id`)
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
CREATE TABLE `t_posts` (
`post_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_user_id` bigint(20) unsigned DEFAULT NULL,
`post_title` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
`post_text` text COLLATE utf8_unicode_ci NOT NULL,
`post_post_id` bigint(20) unsigned DEFAULT NULL,
`post_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
`post_board_id` int(10) unsigned NOT NULL DEFAULT '0',
`post_system_id` int(10) unsigned NOT NULL DEFAULT '0',
`post_deleted` int(2) NOT NULL DEFAULT '0',
PRIMARY KEY (`post_id`)
) ENGINE=InnoDB AUTO_INCREMENT=205 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
INSERT INTO `t_board_categories` (`bc_id`, `bc_name`, `bc_prio`, `bc_system_id`, `bc_deleted`) VALUES (1, 'Test-Category', '1', '1', '0');
INSERT INTO `t_boards` (`board_id`, `board_bc_id`, `board_name`, `board_subtitle`, `board_prio`, `board_system_id`, `board_deleted`) VALUES (1, '1', 'Test-Board', 'Board Subtitle', '1', '1', '0');
INSERT INTO `t_posts` (`post_id`, `post_user_id`, `post_title`, `post_text`, `post_post_id`, `post_date`, `post_board_id`, `post_system_id`, `post_deleted`) VALUES
(1, 14, 'Thread 1', 'Test1', NULL, '2019-05-22 00:18:25', 1, 1, 0),
(2, 14, 'Thread 2', 'Test2', NULL, '2019-05-22 00:18:44', 1, 1, 0),
(3, 14, 'Thread 3', 'Test 3', NULL, '2019-05-22 00:18:51', 1, 1, 0),
(4, 19, 'Thread 4', 'Test 4', NULL, '2019-05-22 00:19:02', 1, 1, 0),
(5, 19, 'Thread 5', 'Test 5', NULL, '2019-05-22 00:19:07', 1, 1, 0),
(6, 19, 'Thread 6', 'Test 6', NULL, '2019-05-22 00:19:15', 1, 1, 0),
(7, 14, 'Reply', 'A', 5, '2019-05-22 00:19:46', 1, 1, 0),
(8, 14, 'Reply', 'B', 5, '2019-05-22 00:19:47', 1, 1, 0),
(9, 14, 'Reply', 'C', 5, '2019-05-22 00:19:49', 1, 1, 0);
Expected Result: One row for thread 1-5 with user-id and date of last reply (userid:14, date: 2019-05-22 00:19:49 for Thread 5, for Thread 1-4 last date is thread-date, userid is thread author userid)
I worked on this the whole evening, would be great to get some help. Thank you in advance!

Answer 1

Your sample data suggests, that post levels cannot span more than two levels (post_post_id cannot be an ID of a post where post_post_id isn't null). Assuming that, you can try to use correlated subqueries getting the sub posts and the post for a post, order them by their date descending and use LIMIT to pick the top one. Also sort the main query by the date of the last sub post descending and use LIMIT to get the top 5. Something along the lines of:
SELECT bc1.bc_id,
bc1.bc_name,
b1.board_id,
b1.board_name,
p1.post_id,
p1.post_title,
(SELECT p2.post_title
FROM t_posts p2
WHERE p1.post_id IN (p2.post_id,
p2.post_post_id)
ORDER BY p2.post_date DESC
LIMIT 1) last_post_title,
(SELECT p2.post_user_id
FROM t_posts p2
WHERE p1.post_id IN (p2.post_id,
p2.post_post_id)
ORDER BY p2.post_date DESC
LIMIT 1) last_post_user_id,
(SELECT p2.post_date
FROM t_posts p2
WHERE p1.post_id IN (p2.post_id,
p2.post_post_id)
ORDER BY p2.post_date DESC
LIMIT 1) last_post_date
FROM t_posts p1
INNER JOIN t_boards b1
ON b1.board_id = p1.post_board_id
INNER JOIN t_board_categories bc1
ON bc1.bc_id = b1.board_bc_id
WHERE p1.post_post_id IS NULL
ORDER BY last_post_date DESC
LIMIT 5;
db<>fiddle