Check Available Rooms For Selected Dates

Hello there,

Working on a small hotel reservation system and got stucked on "check availability" part and need your help.

My DB Structure : 

1- ROOMS table :
    room_id  - room_no   -  room_capacity  

2- RESERVATION table :
    reservation_id - days - daily_price - room_no - customer_id 

room_no is the FK.

I can get the result of the rooms which are booked on selected dates but I cannot get the ones which are available (which are not booked for those days).

I have tried lots of things upon my searchs but let me write here some example : 

My Modal :

PHP Code:

public function check_room_availability($date_in, $date_out) {

        $query = $this->db->select('*')->from('rooms')
        ->join('reservation', 'rooms.room_no = reservation.room_no', 'LEFT')
        ->where('reservation.days >=', $date_in)
        ->where('reservation.days <=', $date_out)
        ->order_by('reservation.room_no', 'ASC')
        ->get()->result();
        
return $query; 


Thanks in advance.