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;