???????????????

???

-- FROM????????????????????????????????????????????????
SELECT VirtualFlightTable.flight_ID
FROM
    (SELECT flight_ID, orig_airport, dest_airport
    FROM Flights
    WHERE (orig_airport = 'SFO' OR dest_airport = 'SCL') )
AS VirtualFlightTable
-- FROM????????????????????????????????????(values???)????????????
SELECT mycol1
FROM
    (VALUES (1, 2), (3, 4))
AS mytable (mycol1, mycol2)
-- EXISTS???????????????????????????
SELECT *
FROM Flights
WHERE EXISTS
    (SELECT * FROM Flights WHERE dest_airport = 'SFO'
    AND orig_airport = 'GRU')
-- IN???????????????????????????
SELECT flight_id, segment_number
FROM Flights
WHERE flight_id IN
    (SELECT flight_ID
    FROM Flights WHERE orig_airport = 'SFO'
    OR dest_airport = 'SCL')
-- ???????????????????????????????????????
SELECT NAME, COMM 
FROM STAFF 
WHERE COMM > 
(SELECT AVG(BONUS + 800)
     FROM EMPLOYEE
     WHERE COMM < 5000)