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
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:
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
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.
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.