Exercise #55 (tips and solutions)
Exercise #55 (tips and solutions)
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;mssql
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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.