Для семи последовательных дней, начиная от минимальной даты, когда из Ростова было совершено максимальное число рейсов, определить число рейсов из Ростова.

Задание: 94 (Serge I: 2003-04-09)

 
Вывод: дата, количество рейсов 

SELECT DATEADD(day, S.Num, D.date) AS Dt,
       (SELECT COUNT(DISTINCT P.trip_no)
        FROM Pass_in_trip P
               JOIN Trip T
                 ON P.trip_no = T.trip_no
                    AND T.town_from = 'Rostov'
                    AND P.date = DATEADD(day, S.Num, D.date)) AS Qty
FROM (SELECT (3 * ( x - 1 ) + y - 1) AS Num
        FROM (SELECT 1 AS x UNION ALL SELECT 2 UNION ALL SELECT 3) AS N1
               CROSS JOIN (SELECT 1 AS y UNION ALL SELECT 2 UNION ALL SELECT 3) AS N2
        WHERE (3 * ( x - 1 ) + y ) < 8) AS S,
       (SELECT MIN(A.date) AS date
        FROM (SELECT P.date,
                       COUNT(DISTINCT P.trip_no) AS Qty,
                       MAX(COUNT(DISTINCT P.trip_no)) OVER() AS M_Qty
                FROM Pass_in_trip AS P
                       JOIN Trip AS T
                         ON P.trip_no = T.trip_no
                            AND T.town_from = 'Rostov'
                GROUP BY P.date) AS A
        WHERE A.Qty = A.M_Qty) AS D

Все задачи и решения: https://exercises-on-sql.blogspot.com/2017/02/select-sql.html