SQL query question: SELECT … NOT IN

SELECT idCustomer FROM reservations
WHERE idCustomer NOT IN
  (SELECT distinct idCustomer FROM reservations
   WHERE DATEPART ( hour, insertDate) < 2)

SELECT idCustomer FROM reservations

SELECT distinct idCustomer FROM reservations
WHERE DATEPART ( hour, insertDate) < 2

SELECT distinct idCustomer FROM reservations
WHERE DATEPART ( hour, insertDate) < 2
  and idCustomer is not null

WHERE field1 NOT IN (1, 2, 3, null)

WHERE NOT (field1 = 1 OR field1 = 2 OR field1 = 3 OR field1 = null)

IF 1 NOT IN (1, 2, 3, NULL) PRINT '1 NOT IN (1, 2, 3, NULL)'
IF 1 NOT IN (2, 3, NULL) PRINT '1 NOT IN (2, 3, NULL)'
IF 1 NOT IN (2, 3) PRINT '1 NOT IN (2, 3)' -- Prints
IF 1 IN (1, 2, 3, NULL) PRINT '1 IN (1, 2, 3, NULL)' -- Prints
IF 1 IN (2, 3, NULL) PRINT '1 IN (2, 3, NULL)'
IF 1 IN (2, 3) PRINT '1 IN (2, 3)'

SELECT idCustomer FROM reservations
EXCEPT
SELECT idCustomer FROM reservations WHERE DATEPART ( hour, insertDate) < 2

SELECT Reservations.idCustomer FROM Reservations (nolock)
LEFT OUTER JOIN @reservations ExcludedReservations (nolock) ON Reservations.idCustomer=ExcludedReservations.idCustomer AND DATEPART(hour, ExcludedReservations.insertDate) < 2
WHERE ExcludedReservations.idCustomer IS NULL AND Reservations.idCustomer IS NOT NULL
GROUP BY Reservations.idCustomer