stuck with notification system - php

Question

I am developing a notification system in PHP, MySql, jquery ajax on a website that i am working on. I have a post which you follow if you comment on it. My problem is that i am stuck i don't know how can i send notification to all the users following the post. I did successfully send notification to the creator or the post though.
in the code i will be showing the query that enters a notification into notifications table, i will post the notifications_table structure and discussion_followers table which include the users following a post
here is the query that enters the notification into the notifications_table
// the board user_id is not the same as the logged in user which means the logged in user is not commenting on his post
// which means we insert notification in table
$query_not = "INSERT INTO notifications ( ";
$query_not .= "from_user, to_user, post_type, post_id, type, comment_id, notification_timestamp";
$query_not .= ") VALUES (";
$query_not .= " $user_id, $discussion_user_id_check, 'discussion', $discussion_id, 'comment', $fetched_comment_id, $discussion_comment_timestamp";
$query_not .= ")";
$result_not = mysqli_query($connection, $query_not);
so what i already achieved is : a user which is not the logged in user comments on a discussion post if he hasn't already followed the post he will will follow by entering his id with the post id into the following table, a new notification is inserted into the notifications_table. The user who created the post will receive this notification. What i want to achieve more is send notifications to all following this post that a new comment has been entered.
The notifications table structure is as follow:
notification_id, from_user, to_user, post_type, post_id, type, comment_id, reply_id, notification_timestamp, viewed
example of notification:
121, 20, 30, discussion, 31, comment, 39, 0, 1534184319, 0
discussions_followers table which follow the discussion post structure is :
follow_id, user_id, discussion_id

Answer 1

Honestly if i want to do what you want, i do not create my data structure like that to fill the purpose. but what ever, you can do a query:
select from_user from notifications where post_id={discussion_id} group by from_user;
then all ids fetched are users you want to push notification from new user. (you can skip $from_user==$user_id). lets say you have $result, then (simple code):
for($i=0, $i<count($result); $i++){
insert into notifications (from_user, to_user) values ($user_id, $result[$i]
['from_user'])
}
what i prefer:
for system like push notification. i always prefer to implement whole system as microservice and outside my project. (just you know not related). but what i do, i would use one of those message queue systems and not store such data in database FOR READ. your table is more like a log table which you can log you notification and write what ever you want to it for further process. but for read from it just to know who should push who, its not efficient. so have table post with 1 unique id and users id involved with that post. on each comment create queue with id "push-$id" and push comment and user id to it one by one. background manager of queues will know how to handle queues with id "push-*". creating queue may be a problem here so you can create empty one on each post created. (its not doing anything cause by that you just create pointer nothing else).