SQL示例查询解决方案（按技术和子查询技术分组）

Question

Tables:

Classes(class, type, country, numGuns, Bore, displacement)
Ships(name, class, launched)

Question: Find for each class the year in the which the first ship of that class was launched?

My Attempt:

         select class, min(launched) from Ships
         group by (class)

Answer provided:

         select class, launched from ships as s1
         where launched <= ALL (select year from ships as s2
                                where s2.class = s1.class)

So my question is, will I get desired result from my query? I don't really understand if it is as complicated as the Answer provided especially when there is no column as year in the table.

The minimum year in the group for class should correspond to the the first ship of that class.

Answer 1

Yes, your answer works.

In fact it is better than the provided answer, because that query actually returns ALL ships and launches in a class, for the first year that that class was launched.

Answer 2

Right solution to this problem.

You're just selecting from Ships table but it's quite possible some classes do not have any ships assigned in Ships table, you need to consider those as well hence using a LEFT JOIN to adhere the condition.

SELECT C.class, 
       CASE WHEN S.launched IS NULL THEN 
       (SELECT MIN(launched) 
        FROM   Ships as S 
       WHERE   S.class = C.class) ELSE S.launched END
FROM   Classes as C LEFT JOIN Ships as S
ON     C.class = S.name;