CREATE TABLE Ship (Ship_ID INT, Name TEXT, Type TEXT, Built_Year FLOAT, Class TEXT, Flag TEXT)
CREATE TABLE captain (Captain_ID INT, Name TEXT, Ship_ID INT, age TEXT, Class TEXT, Rank TEXT)

What is the average age of captains in different classes?, and order from low to high by the x-axis.
SELECT Class, AVG(age) FROM captain GROUP BY Class ORDER BY Class
SELECT "Class", AVG("age") FROM "captain" GROUP BY "Class" ORDER BY "Class" NULLS FIRST