I have a problem with mysql alias. 

I have this query:

```
SELECT (`number_of_rooms`) AS total, id_room_type, 
     COUNT( fk_room_type ) AS reservation , 
     SUM(number_of_rooms - reservation) AS result
FROM room_type
    LEFT JOIN room_type_reservation 
         ON id_room_type = fk_room_type
WHERE  result > 10
GROUP BY id_room_type

```

My problem start from `SUM`, `cannot recognize reservation` and then i want to use the result for a where condition. Like (`where result > 10`)