I have MySQL table that is used to collect letter data by the subject_id, joined with tbl_user table. tbl_user as follows :

+--------+------------+----------+
| userID | subject_id | username |
+--------+------------+----------+
|      1 | 100        | user1    |
|      2 | 101        | user2    |
|      3 | null       | user3    |
|      4 | 102        | user4    |
+--------+------------+----------+

In my controller, I used the following code to filter records by subject_id.

$subject = $this->session->userdata('subject_id');
        if ($subject && $subject != null )
            $this->datatables->where('letter_letter.subject', $subject);

This is working perfectly. In the other words, letter records filtering by subject_id fine. But I want to list all the records in the letter_letter table for user, "user3" in the session.

How can I modify my code to do this ?. Can anyone help me ?