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)

Stacked bar of class and the number of class colored by Rank
SELECT Class, COUNT(Class) FROM captain GROUP BY Rank, Class
SELECT "Class", COUNT("Class") FROM "captain" GROUP BY "Rank", "Class"