DOUBLE NOT IN() IN NESTED QUERIES : SELECT S.sname
FROM Sailors S
WHERE S.sid NOT IN (SELECT R.sid
        FROM Reserves R
        WHERE R.bid NOT IN (SELECT B.bid    
        FROM Boats B
        WHERE B.color='red'))
Trying to understanding this. How does this piece of code finds names of sailors who reserved only red boats and sailors that do not reserve any boat at all. How does a NOT IN nested within another NOT IN works?