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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 #46 page 1

Point out the names, displacements and number of guns of ships participating in the battle at Guadalcanal.

The all ships which had taken part in the battle of Guadalcanal is in the Outcomes table, and all the characteristics is in the Classes table. The first occured idea is in the joining of this tables:

Solution 3.1.1

Console

Execute

SELECT Outcomes.ship, Classes.displacement, Classes.numGuns 
FROM Classes RIGHT JOIN
 Outcomes ON Classes.class = Outcomes.ship 
WHERE Outcomes.battle = 'Guadalcanal';

The outer join is using correctly, because task says about ships which had taken part in battle, so we need to select all ships, no matter the name matches the class name or it does not. Note that the inner join returns empty set, because there is no lead ships in the main database which take part in this battle. So we have: