Stuck in fetching specific data

Hello to everyone! I'm creating my own app, where people can share own usernames. I have a problem here. Everthing is fine, but i'm unable to fetch data correctely.

What i want to do is:

• grub from my database_name where people have a specific platform_name
  

So basically if people match Telegram, this query will output only those people. 

I'm using this to get usernames but now i need to filter them:

PHP Code:

        public function get_usernames($slug = FALSE, $limit = FALSE, $offset = FALSE){
            if ($limit) {
                $this->db->limit($limit ,$offset);
            }
            if($slug === FALSE){
                $this->db->order_by('usernames.id', 'DESC');
                $query = $this->db->get('usernames');
                return $query->result_array();
            }

            $query = $this->db->get_where('usernames', array('slug' => $slug));
            return $query->row_array();
        }