The next solution is correctly (but excessively) takes into account ships from Outcomes table.
SELECT t1.class, MIN(b.launched) FROM (SELECT name, class FROM Ships UNION SELECT ship, class FROM Outcomes JOIN Classes ON ship = class AND ship NOT IN(SELECT name FROM Ships ) ) t1 LEFT JOIN Ships b ON t1.class = b.class GROUP BY t1.class;
The only mistake of this solution occures in case when there is a class without ships in current state of DB. Remark that such situation is possible, because tables Classes and Ships connected by one-to-many relationships.
To return to discussion of exercise #55
To solve a problem on SQL-EX.RU
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