How to create a MySQL query that returns the replies to a given user's comments

Question

I am trying to create a page that displays all replies to first tier comments of the active user. Here is an example DB table similar to the one I have...

commentid	postid	commid	username	userid	commtext
52	100	0	user1	1	first
53	100	52	user2	2	second
54	100	52	user1	1	third

"commentid" is the comment's unique id, "postid" is the id of the post it was on, "commid" gives the number of the comment that it was a response to ("0" indicates it is on the first tier), and so on...

If I am logged in as "user1" and I do the following...

$ra = $DB->Execute('SELECT * FROM comments WHERE userid = '.$DB->qstr($id).' ORDER BY commentid DESC')

...it will return comments #52 and #54. Simple enough.

I am having an issue creating a request to the DB that will output a list that gives me #53 (which was a reply to user1's comment). If possible, I do not want to include #54 which was written by the active user him/herself.

What would be a good way to construct this query?

Answer 1

If you just want to retrieve the first level replies (ignoring those from the initial user), you can JOIN comments to itself to find the replies, filtering out those with the same userid :

SELECT rep.*
FROM comments rep
JOIN comments cmt
WHERE rep.commid = cmt.commentid
  AND cmt.userid = 1
  AND rep.userid != 1

Output (for your sample data):

commentid   postid  commid  username    userid  commtext
53          100     52      user2       2       second

Demo on db-fiddle