Order by in a self referencing table

Question

I have a table of comments (to some article like this):

---------------------------------------------------------------
id  |comment_id     |likes_count      |add_time
---------------------------------------------------------------
0      NULL            0                time0
1      NULL            2                time1
2      0               0                time2
3      0               0                time3
4      NULL            1                time4
5      1               0                time5

The comment_id is referencing to the id in the same table. When comment_id is NULL , this means that this comment is parent (it is not a response to another comment, but rather to the article itself), but if it is not null, it means that this comment is a child (it is a response to another comment in the article).

The query I am using orders the results in a manner where each child is displayed under its parent.

SELECT *
FROM comments
ORDER BY
    COALESCE (comment_id, id) DESC, 
    (comment_id IS NULL) DESC, 
    likes_count DESC

But I can't figure out how to make parent comments (with their children following it) with the most likes_count appear first in the results, the likes_count DESC seems to have no effect, as results are always ordered by the id .

Result returned by the query:

---------------------------------------------------------------
id  |comment_id     |likes_count      |add_time
---------------------------------------------------------------
0      NULL            0                time0
2      0               0                time2
3      0               0                time3
1      NULL            2                time1
5      1               0                time5
4      NULL            1                time4

Expected result:

---------------------------------------------------------------
id  |comment_id     |likes_count      |add_time
---------------------------------------------------------------
1      NULL            2                time1
5      1               0                time5
4      NULL            1                time4
0      NULL            0                time0   
2      0               0                time2
3      0               0                time3

phpMyAdmin show the following info:

Server type: MariaDB

Server version: 10.1.40-MariaDB-cll-lve - MariaDB

Server Protocol version: 10

cpsrvd 11.78.0.34

Database client version:libmysql - 5.1.73

PHP version: 7.2.7

phpMyAdmin: 4.8.3

Answer 1

You could try adding a sort level using the max likes count over each parent child group:

SELECT *
FROM comments
ORDER BY
    MAX(likes_count) OVER (PARTITION BY COALESCE(comment_id, id)) DESC,
    COALESCE (comment_id, id) DESC, 
    comment_id IS NULL DESC;

If your database (whatever that actually is) does not support analytic functions, then we can do the same logic with a join:

SELECT c1.*
FROM comments c1
INNER JOIN comments c2
    ON COALESCE(c1.comment_id, c1.id) = c2.id
ORDER BY
    c2.likes_count DESC,
    COALESCE (c1.comment_id, c1.id) DESC, 
    c1.comment_id IS NULL DESC;