Question: What is the quantity preserved when 4 were made of class F-21?
SQL: SELECT "Quantity preserved" FROM "2-17248696-6" WHERE "Quantity made" = '4' AND "Class" = 'F-21';

Question: What fleet number had 3 made?
SQL: SELECT "Fleet number(s)" FROM "2-17248696-6" WHERE "Quantity made" = '3';

Question: What class had 1 made and fleet number of 406?
SQL: SELECT "Class" FROM "2-17248696-6" WHERE "Quantity made" = '1' AND "Fleet number(s)" = '406';

Question: What class has 0 preserved and 2 made?
SQL: SELECT "Class" FROM "2-17248696-6" WHERE "Quantity preserved" = '0' AND "Quantity made" = '2';