CREATE TABLE captain (captain_id DECIMAL, name TEXT, ship_id DECIMAL, age TEXT, class TEXT, rank TEXT)
CREATE TABLE ship (ship_id DECIMAL, name TEXT, type TEXT, built_year DECIMAL, class TEXT, flag TEXT)

Return the flag that is most common among all ships.
SELECT flag FROM ship GROUP BY flag ORDER BY COUNT(*) DESC LIMIT 1
SELECT "flag" FROM "ship" GROUP BY "flag" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1