Binary Tree Get All Left / Right Nodes

Question

Here's my database structure

id | parent_id | depth | position
---------------------------------
2  | 1         | 1     | left
---------------------------------
3  | 1         | 1     | right
---------------------------------
4  | 2         | 2     | left
---------------------------------
5  | 3         | 2     | left
---------------------------------
6  | 2         | 3     | right
---------------------------------
7  | 4         | 4     | right
---------------------------------

I wrote a recursive function to get the total left or right nodes of a particular "id" on a particular depth, but the it is too heavy as the depth can go up to 300+.

For example i want to get how many child nodes on the left section does id 2 has, in this case 2, 4 and 7 is on the left section of node 2, and on particular depth 4 there is only 1 on the left section.

Is there a better way to get all the left section node count on a particular depth.

protected function rec($parent_id, $current_level, $requested_level, $created_at) {

        $query = DB::select("select username, id, depth, DATE(created_at) as created_at from users where parent_id = $parent_id");

        if ($current_level == $requested_level) {                                    
            $this->result1++;
            return;
        }        
        else {
            if(!empty($query)){
                foreach($query as $row){
                    $this->rec($row->id, $row->depth, $requested_level, $row->created_at);                     
                }
            }
            else{                
                return;
            }                    
        }
    }