SELECT Rank, COUNT(*) FROM captain WHERE age < 50 GROUP BY Rank
Count the number of captains younger than 50 of each rank.
CREATE TABLE Ship (
    Ship_ID int,
    Name text,
    Type text,
    Built_Year real,
    Class text,
    Flag text
)

CREATE TABLE captain (
    Captain_ID int,
    Name text,
    Ship_ID int,
    age text,
    Class text,
    Rank text
)