MySQL LEFT JOIN LIMIT for each row [duplicate] - mysql

Question

This question already has answers here:
Using LIMIT within GROUP BY to get N results per group?
(14 answers)
Closed 2 years ago.
I would like to get a list of Posts including the last 2 comments for each particular post (if any).
I have been using the query below, that returns all the comments for each post, however it has a limitation, because the GROUP_CONCAT() length is limited to 1024 characters by default, so if there are a lot of comments on a post, the 'comments' value will be cut off, and it won't be a valid JSON output anymore.
SELECT post.*,
CONCAT('[', GROUP_CONCAT(DISTINCT JSON_OBJECT(
'id', postComment.id,
'date', postComment.date,
'content', postComment.content,
'user', CONCAT(postComment.fName, postComment.lName)
) ORDER BY postComment.id DESC),']') AS comments
FROM posts AS post
LEFT JOIN (
SELECT comment.*, commentUser.fName, commentUser.lName
FROM comments AS comment
LEFT JOIN users AS commentUser ON comment.uID = commentUser.id
ORDER BY comment.id DESC
) AS postComment ON postComment.postID = post.id
WHERE post.uID = 37
GROUP BY post.id
ORDER BY post.id DESC
LIMIT 0,5;
Due to this limitation, I was thinking of returning only the last 2 comments for each post by adding LIMIT 0,2 to the LEFT JOIN SELECT clause like this:
SELECT post.*,
CONCAT('[', GROUP_CONCAT(DISTINCT JSON_OBJECT(
'id', postComment.id,
'date', postComment.date,
'content', postComment.content,
'user', CONCAT(postComment.fName, postComment.lName)
) ORDER BY postComment.id DESC),']') AS comments
FROM posts AS post
LEFT JOIN (
SELECT comment.*, commentUser.fName, commentUser.lName
FROM comments AS comment
LEFT JOIN users AS commentUser ON comment.uID = commentUser.id
ORDER BY comment.id DESC
LIMIT 0, 2
) AS postComment ON postComment.postID = post.id
WHERE post.uID = 37
GROUP BY post.id
ORDER BY post.id DESC
LIMIT 0,5;
But now it returns the first two comment for the very first post only...
Could anyone tell me how can I get this work correctly, so the query results return the first two comments FOR EACH particular post and why this is happening?
Cheers!

Answer 1

As a starter: if your actual problem is the limitation on the number of characters returned by GROUP_CONCAT(), you can increase it by modifying session variable group_concat_max_len, like so:
SET SESSION group_concat_max_len = 1000000;
Then you can run your query (in the same session).
As for your question itself: I would recommend turning the LEFT JOIN to a correlated subquery; basically the subquery is re-executed for each and every post, so you can use ORDER BY and LIMIT:
SELECT p.*,
CONCAT(
'[',
(
SELECT GROUP_CONCAT(
JSON_OBJECT(
'id', c.id,
'date', c.date,
'content', c.content,
'user', CONCAT(u.fName, u.lName)
)
ORDER BY c.id DESC
)
FROM comments AS c
LEFT JOIN users AS u ON c.uID = u.id
WHERE c.postID = p.id
ORDER BY c.date DESC
LIMIT 2
),
']'
) AS comments
FROM posts AS p
WHERE p.uID = 37
GROUP BY p.id
ORDER BY p.id DESC
LIMIT 0,5;
As a final thought: do upgrade to MySQL 5.7.22 or higher! Then you can use JSON_ARRAYAGG(), and you won't have to worry about this all anymore.