LIMIT MySQL subquery using variable value before LEFT JOIN - mysql

Question

I have this MySQL table posts:
id | content | parentid | userid
--------------------------------------
01 | Post test #1 | 0 | 1
02 | Post test #2 | 0 | 1
03 | Comment #1 | 1 | 2
04 | Comment #2 | 1 | 1
05 | Post test #3 | 0 | 3
06 | Comment #3 | 1 | 2
07 | Comment #4 | 2 | 5
08 | Comment #5 | 5 | 6
09 | Comment #6 | 1 | 4
10 | Post test #4 | 0 | 4
This is just an example for stackoverflow
Now I need to LIMIT comments for each post and so far I have used this query:
SELECT
`posts`.`id` AS `post_id`,
`posts`.`content` AS `post_content`,
`posts`.`parentid` AS `post_parentid`,
`posts`.`userid` AS `post_userid,
`comments`.`id`, 0 AS `comment_id`,
`comments`.`content` AS `comment_content`,
`comments`.`parentid` AS `comment_parentid`,
`comments`.`userid` AS `comment_userid,
IF( IFNULL( `comments`.`id`, 0 ) > 0, "comment", "post" ) AS `contenttype`
FROM `posts` AS `posts`
LEFT JOIN ( SELECT "" AS `hello` ) AS `useless` ON #pid := `posts`.`id`
LEFT JOIN ( SELECT
`posts`.`id` AS `id`,
`posts`.`id` AS `id`,
`posts`.`id` AS `id`,
`posts`.`id` AS `id`
FROM `posts`
WHERE `posts`.`parentid` = #pid
LIMIT 10
) AS `comments`ON `comments`.`parentid` = `posts`.`id`
WHERE
`posts`.`userid` = {USERID}
To archive this I have joined an useless "table" just to update #pid (parentid) variable.
Is this the only way to LIMIT subquery results? I don't like the idea of that useless JOIN.
What if I have to LIMIT posts in the example above without affecting the comments LIMIT. Can you please give me a better query?

Answer 1

The real reason of posting this question was to load 10 comments with 10 sub-comments for each comment. On the question I have asked to load posts & comments so the idea is the same.
The example posted in my question doesn't works because the subquery will executed before the variable #pid gets updated.
Because I'm using PHP then I'm posting here the solution in MySQL & PHP for this situation.
1 - First let's load posts with this SQL query
SELECT
`posts`.`id` AS `id`,
`posts`.`content` AS `content`,
`posts`.`parentid` AS `parentid`,
`posts`.`userid` AS `userid
FROM `posts` AS `posts`
WHERE
`posts`.`userid` = {USERID}
AND
`posts`.`parentid` = '0'
ORDER BY `posts`.`id` DESC
LIMIT 10
2 - Store posts information in $posts array:
$posts = [];
while ( $row = $result->fetch_object() )
{
$posts[] = (object) [ "id" => $row->id,
"content" => $row->content,
"userid" => $row->userid,
"comments" => []
];
}
3 - Prepare SQL to load comments:
$size = count( $posts );
$sql = "";
for ( $i = 0; $i < $size; $i ++ )
{
$sql .= ( $sql != "" ? "UNION ALL " : "" )
. "( "
. "SELECT "
. "`comments`.`id` AS `id`, "
. "`comments`.`content` AS `content`, "
. "`comments`.`parentid` AS `parentid`, "
. "`comments`.`userid` AS `userid "
. "FROM `posts` AS `comments` "
. "WHERE "
. "`comments`.`parentid` = '" . $post[ $i ]->id . "' "
. "ORDER BY `comments`.`id` ASC "
. "LIMIT 10 "
. ") ";
}
4 - After executing the $sql code let's store comments for each post:
while ( $row = $result->fetch_object() )
{
$posts[ $row->parentid ]->comments[] = (object)[
"id" => $row->id,
"content" => $row->content,
"userid" => $row->userid,
];
}
As you can see this can be used for also comments (instead of posts ) & sub-comments (instead of comments). MySQL variables are not helpful this time. Of course to create a pagination you have to add additional field (replies) in the table and update that during comment creation.
If someone has a better solution is welcomed.