Exercise #70 (tips and solutions)

SELECT q.battle
FROM (
    --Determine ships from Ships table which taking part in battles
    SELECT Outcomes.battle, Outcomes.ship, Classes.country
    FROM Classes 
        INNER JOIN Ships ON Classes.class = Ships.class 
        INNER JOIN Outcomes ON Ships.name = Outcomes.ship
    UNION
    --Determine leading ships from the Outcomes table
    SELECT Outcomes.battle, Outcomes.ship, Classes.country
    FROM Outcomes 
        INNER JOIN Classes ON Outcomes.ship = Classes.class
    ) as q
GROUP BY q.battle
HAVING COUNT(q.country) >= 3;