example.sql

SELECT DISTINCT outlet_name,
                IFNULL(SUM(reservation_pax), 0) AS reserved,
                outlet_max_capacity,
                ( outlet_max_capacity - IFNULL(SUM(reservation_pax), 0) ) AS available
FROM   `outlets`
       LEFT JOIN `reservations`
         ON `reservations`.reservation_outlet_id = `outlets`.outlet_id
            AND reservation_date = '2011-09-22'
            AND HOUR(reservation_time) = '17'
GROUP  BY outlet_id 

query.sql

SELECT DISTINCT outlet_name,
                IFNULL(SUM(reservation_pax), 0) AS reserved,
                outlet_max_capacity,
                ( outlet_max_capacity - IFNULL(SUM(reservation_pax), 0) ) AS available
FROM   `outlets`
       LEFT JOIN `reservations`
         ON `reservations`.reservation_outlet_id = `outlets`.outlet_id
            AND reservation_date = CURDATE()
            AND HOUR(reservation_time) = HOUR(CURTIME())
GROUP  BY outlet_id