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SQL Problems and solutions S. I. Moiseenko

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Common mistakes in solving SELECT statement exercises

Database «Ships»

Exercise #151 page 1

Define the names of all the ships in the database which were definitely launched before 1941.

Despite of complexity level of 2, the solution of this exercise have caused, apparently, the greatest difficulties. And these are not due to complexity in construction of query, but due to logic of the solution of a problem.

Until recently this task has been formulated as: «Define the names of all ships, which were launched before 1918.». It seemingly has no difference, hasn't it? Does the following decision not be a correct?

Solution 3.5.1

Console

Execute

SELECT name AS shipName
FROM Ships
WHERE launched < 1941;

No. As it has fairly been noticed by visitors of a site, dates of battles are not considered here in any way. Really, the ship is necessary to include into the result set, if the year of launching of the ship, which took part in the battle before 1941, is unknown. In the initial formulation of this problem where year 1918 was presented, there was possible to not care about it, as the database formally contains the information on battles of the World War II which has begun in 1939.

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SQL Problems and solutions

S. I. Moiseenko

Common mistakes in solving SELECT statement exercises

Database «Ships»

Exercise #151 page 1