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

A stacked bar graph shows the total number of captains categorize by ranks of different classes.
SELECT Class, COUNT(Class) FROM captain GROUP BY Rank, Class
SELECT "Class", COUNT("Class") FROM "captain" GROUP BY "Rank", "Class"