SELECT table 2 insite a COUNT comparing to table 1 - sql

Question

I have four tables: SCHED_FLIGHT, AIRCRAFT, PLANETYPE and RESERVATIONS. I want to do something like this:
select S.SCHED_NO as "Scheduled Flight Number", P.CAPACITY - COUNT (SELECT * FROM RESERVATIONS R WHERE R.SCHED_NO = S.SCHED_NO) "Remaining Seats"
from PLANETYPE P, AIRCRAFT A, SCHED_FLIGHT S
where S.SERIAL_NO = A.SERIAL_NO and A.TYPE_NO = P.TYPE_NO;
As you can see, each SCHEDULED_FLIGHT has a AIRCRAFT SERIAL_NO, each AIRCRAFT has a PLANETYPE TYPE_NO and each PLANETYPE has a different capacity, so, each SCHED_FLIGHT has a capacity based on the plane and I want to get the number of remaining seats counting the number of RESERVATIONS made to that flight.
Of course the code doesn't work, but I have no idea how to solve this problem. Any tips?
edit: I've received some answers, but first I was just showing two tables as an example - but actually there's 4 tables involved and I would have to include a where clause or multiple joins... so I'm still confused. What should I do now? Look at my code.

Answer 1

Update:
I'm not sure that all of columns correspond to your real columns, but this query might look like this:
SELECT
SF.SCHED_NO AS 'Scheduled Flight Number',
SF.CAPACITY - COUNT (R.SCHED_NO) AS 'Remaining Seats'
FROM
(
SELECT
SF.SCHED_NO,
P.CAPACITY
FROM SCHEDULED_FLIGHT SF
INNER JOIN AIRCRAFT A
ON SF.AIRCRAFT_SERIAL_NO = A.SERIAL_NO
INNER JOIN PLANETYPE P
ON A.PLANETYPE_NO = P.PLANETYPE_NO
) SF
LEFT JOIN RESERVATIONS R
ON SF.SCHED_NO = R.SCHED_NO
GROUP BY SF.SCHED_NO, SF.CAPACITY

Answer 2

I would be inclined to approach this as an aggregation with a join. First, aggregate the reservations by flight number. Then join that back to FLIGHT and calculate the remaining capacity:
SELECT F.FLIGHT_NO, (F.CAPACITY - COALESCE(cnt, 0)) as Remaining
FROM FLIGHT F LEFT JOIN
(SELECT R.FLIGHT_NO, COUNT(*) as cnt
FROM RESERVATIONS R
GROUP BY R.FLIGHT_NO
) R
ON R.FLIGHT_NO = F.FLIGHT_NO;