I have problems with solving a sql question. There is an assumption:
The database of naval ships that took part in World War II is under consideration. The database has the following relations:
Classes(class, type, country, numGuns, bore, displacement)
Ships(name, class, launched)
Outcomes(ship, battle, result)
Ships in classes are arranged to a single project. A class is normally assigned the name of the first ship in the class under consideration (head ship); otherwise, the class name does not coincide with any ship name in the database.
The Classes relation includes the class name, type (bb for a battle ship, or bc for a battle cruiser), country where the ship was built, number of main guns, gun caliber (diameter of the gun barrel, in inches), and displacement (weight in tons). The Ships relation includes the ship name, its class name, and launch year. The Battles relation covers the name and date of a battle the ships participated; while the result of their participation in the battle (sunk, damaged, or unharmed - OK) is in the Outcomes relation. Note: the Outcomes relation may include the ships not included in the Ships relation.
And there is a question:
The weight of a gun missile (in pounds) is almost equal to one-half cube of its caliber (in inches). Define the average weight of the missile for the ships of each country (taking into account Outcomes table).
I have written the following query which does not work correctly:
select country,avg(0.5*(bore*bore*bore)) from classes right join (select class from ships union all select ship from outcomes join classes on outcomes.ship = classes.class) T on classes.class = T.class group by country
the number of question is 32(3).