Common mistakes in solving SELECT statement exercises

Tips and solutions

It's an awful trouble with this head ships! Below we see how wrong solution has gotten for the task about the head ships, which were launched before 1941.

Console

Execute

SELECT class 
FROM Classes 
WHERE EXISTS (SELECT 1 
 FROM Ships 
 WHERE launched < 1941 AND 
 Ships.class = Classes.class
 );

SELECT class FROM Classes WHERE EXISTS (SELECT 1 FROM Ships WHERE launched < 1941 AND Ships.class = Classes.class );

    Scheme

I.e. class here is identified with presence of the head ship in the database, namely, the class, which include a ship launched before 1941, is being found. As it is said in the database definition, head ship is the ship with name matching with the name of a class. But that not always exist a ship the name of which coincides with a class name. Therefore to search for the head ships that are absent from Ships table follows exclusively in Outcomes table.

At last, about variant unconsidered in the solution 5.3.3. So, the next situation is possible. There is a head ship with unknown launch year. Moreover, it could take part in the only one battle, for example in 1945 (i.e. after 1941). Let all the ships of the same class have unknown launch year (NULLs are allowed). But if at least one of these ships took part in battle before 1941, we should include both of the ships into the result set, because the head ship (if it exists!) is launched before any other ship in its class.

Here is the solution which, apparently, considers all the stipulated moments:

Console

Execute

-- Ships launched before 1941
SELECT name 
FROM Ships 
WHERE launched < 1941
 
UNION 
-- Ships participated in battles before 1941
SELECT ship 
FROM Outcomes JOIN 
 Battles ON Battles.name = Outcomes.battle 
WHERE date < '19410101'
 
UNION 
-- Head ships in Outcomes, class of which includes other ships 
-- launched before 1941
SELECT ship 
FROM Outcomes 
WHERE ship IN (SELECT class 
 FROM Ships
 WHERE launched < 1941
 )
 
UNION 
-- Head ships in Outcomes when at least one ship from 
-- the same class participated in a battle before 1941
SELECT ship 
FROM Outcomes 
WHERE Ship IN (SELECT class 
 FROM Ships JOIN
 Outcomes ON Ships.name = Outcomes.ship JOIN 
 Battles ON Battles.name = Outcomes.battle 
 WHERE date < '19410101'
 );

-- Ships launched before 1941 SELECT name FROM Ships WHERE launched < 1941 UNION -- Ships participated in battles before 1941 SELECT ship FROM Outcomes JOIN Battles ON Battles.name = Outcomes.battle WHERE date < '19410101' UNION -- Head ships in Outcomes, class of which includes other ships -- launched before 1941 SELECT ship FROM Outcomes WHERE ship IN (SELECT class FROM Ships WHERE launched < 1941 ) UNION -- Head ships in Outcomes when at least one ship from -- the same class participated in a battle before 1941 SELECT ship FROM Outcomes WHERE Ship IN (SELECT class FROM Ships JOIN Outcomes ON Ships.name = Outcomes.ship JOIN Battles ON Battles.name = Outcomes.battle WHERE date < '19410101' );

    Scheme

However the system all the same informs on a mistake …

As it has already been noted in item 3.5, the head ships with unknown launch year can be not only in Outcomes table, but also in Ships table. Moreover, such ships will not be considered by above query if they are absent from Outcomes table, i.e. either they did not participate in battles or the information on their participation is unknown.

Thus it is a pure logic, nothing more.

To return to discussion of exercise #151