How to select NOT using SQL JOIN - sql

Question

My question is to select pilots that are currently not assigned to any flight.
My idea is to use LEFT JOIN to select pilots that are currently assigned to flights, then select the null values. Am I right?
I am trying to select the null value by using WHERE...IS NULL, but I don't know how. Thanks for the help.
There are five tables:
Flight:
PRIMARY KEY (id),
FOREIGN KEY (route) REFERENCES Route (id),
FOREIGN KEY (aircraft) REFERENCES Aircraft (id)
Aircraft:
PRIMARY KEY (id),
FOREIGN KEY (aircraftType) REFERENCES AircraftType (id)
AircraftType:
PRIMARY KEY (id)
Pilot:
PRIMARY KEY (person, forAircraftType),
FOREIGN KEY (person) REFERENCES Person (id),
FOREIGN KEY (forAircraftType) REFERENCES AircraftType (id)
Person:
PRIMARY KEY (id)
Here is my code, I can select pilots that are currently assigned to flight
SELECT name
FROM Flight
LEFT JOIN Aircraft ON Flight.aircraft = Aircraft.id
LEFT JOIN AircraftType ON Aircraft.AircraftType = AircraftType.id
LEFT JOIN Pilot ON AircraftType.id = Pilot.forAircraftType
LEFT JOIN Person ON Pilot.person = Person.id

Answer 1

Instead of trying to join your "Flight" table into the query, simply check if the pilot exists in the Flight table:
SELECT name
FROM Aircraft
LEFT JOIN AircraftType
ON Aircraft.AircraftType = AircraftType.id
LEFT JOIN Pilot
ON AircraftType.id = Pilot.forAircraftType
LEFT JOIN Person
ON Pilot.person = Person.id
WHERE NOT EXISTS (SELECT 1 FROM Flight WHERE Flight.aircraft = Aircraft.id)

Answer 2

Reverse your query or use Right Join as opposed to Left. In its current configuration, Flight is your base table that all other tables are being joined to so it will only return records where pilots have been assinged to a flight i.e. this acts a filter which excludes unassigned Pilots.
By making Pilot or Person as your base, you are aligning all other tables to it and can simply use IS NULL to get your results.
SELECT name
FROM Person
LEFT JOIN Pilot ON Person.id = Pilot.person
LEFT JOIN AircraftType ON Pilot.forAircraftType = AircraftType.id
LEFT JOIN Aircraft ON AircraftType.id = Aircraft.AircraftType
LEFT JOIN Flight ON Aircraft.id = Flight.aircraft
Where Flight.aircraft IS NULL

Answer 3

You want to select those rows which are contains null into name field of flight
SELECT name
FROM Flight
LEFT OUTER JOIN Aircraft
ON Flight.aircraft = Aircraft.id
LEFT JOIN AircraftType
ON Aircraft.AircraftType = AircraftType.id
LEFT JOIN Pilot
ON AircraftType.id = Pilot.forAircraftType
LEFT JOIN Person
ON Pilot.person = Person.id
WHERE Flight.name is null