Common mistakes in solving SELECT statement exercises

Database «Airport»

Among the passengers, who flew by the planes of more then one company, find those who made the same number of trips by the planes of each of these companies. Result set: passenger name.

This task generates a big number of wrong solutions, which I further subdivide into two groups. The first type of error is caused by misinterpreting of the task. For example, some visitors try to identify two passengers, who would
have made equal number of flights with two or more airlines.

I want to make it clear, that it is necessary to consider an individual passenger and to count the number of flights, he has made with each of the airlines, the services of which he has used.

What comes next? Let’s consider an example from the second group of wrong solutions:

Console

Execute

SELECT DISTINCT name
 FROM (SELECT id_psg, id_comp, COUNT(pt.trip_no) AS CNT
             FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no
             GROUP BY id_comp,id_psg)a,
           (SELECT id_psg, id_comp, COUNT(pt.trip_no) AS CNT
            FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no
            GROUP BY id_comp,id_psg)b,
           passenger p
 WHERE a.id_psg=b.id_psg AND a.id_comp<>b.id_comp AND a.cnt=b.cnt
 AND p.id_psg=b.id_psg;

SELECT DISTINCT name  FROM (SELECT id_psg, id_comp, count(pt.trip_no) as CNT              FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no              GROUP BY id_comp,id_psg)a,            (SELECT id_psg, id_comp, count(pt.trip_no) as CNT             FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no             GROUP BY id_comp,id_psg)b,            passenger p  WHERE a.id_psg=b.id_psg and a.id_comp<>b.id_comp and a.cnt=b.cnt  and p.id_psg=b.id_psg;

    Scheme

No scheme

I want to point it out, that the erroneous DISTINCT name eliminates the possibility of taking namesakes into account from the start. Although that’s not the most important thing about it. Two identical queries are joined in FROM clause

Console

Execute

SELECT id_psg, id_comp, COUNT(pt.trip_no) AS CNT
FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no
GROUP BY id_comp,id_psg;

SELECT id_psg, id_comp, count(pt.trip_no) as CNT FROM pass_in_trip pt JOIN trip t ON pt.trip_no=t.trip_no GROUP BY id_comp,id_psg;

    Scheme

No scheme

As it is mentioned above, they are aimed to count for each passenger the number of flights, he has made with the planes of each company.

• the passenger is the same;
• the companies are different;
• the number of flights coincides.

Thus, if the passenger has made 3 flights, say, with Aeroflot Company and 3 flights with Don_avia Company, such a passenger satisfies the terms of such a joining and his name will be returned within the results of such a query. If the passenger has used only two companies, it is the correct result. But what if he has used three companies?

If the result of the aforementioned subquery is

Thus, the number of flights of a passenger with each of the companies, the services of which he has used, must be in proportion 1:1:...:1.