How to count unanswered threads mysql/eloquent - php

Question

Hi I have table threads and questions with relation one-to-many.
every message has foreign key thread_id.
I want to check is last message in the thread is from user and if yes then count it, so it will count all unanswered threads by admin.
+----+-------------+---------+----------+------+
| id | thread_id | user_id | admin_id | text |
| 1 | 1 | 1 | NULL | test |
| 2 | 1 | 1 | NULL | test |
| 3 | 1 | 1 | NULL | test |
| 4 | 1 | 1 | NULL | test |
+----+-------------+---------+----------+------+
The result from this dataset should be 1, because all mesages belongs to the same thread and there is no admin answer (last row has admin_id = NULL).
I tried this:
ThreadMessage::where('admin_id', '=', null)->groupBy('thread_id')->orderBy('id', 'DESC')->limit(1)->count();
Sadly this just counts all the messages where admin_id is null, not just the last ones in thread.
The answer can be in mysql OR laravel eloquent. Thank you very much.

Answer 1

To check for null you need to use IS NULL not = NULL operator. Use whereNull() method
ThreadMessage::whereNull('admin_id')->groupBy('thread_id')->count();
EDIT
To check if the last id is reponded to by admin, you can do it in two ways
with code
$thread = ThreadMessage::orderBy('thread_id', 'desc')->groupBy('thread_id')->first();
$answeredByAdmin = $thread->admin_id != null;
With query (assuming the table name is thread_messages)
ThreadMessage:::whereNull('admin_id')
->groupBy('thread_id')
->whereIn('id', \DB::raw('(SELECT MAX(id) FROM thread_messages GROUP BY question_id)'))
->count();