SQL SUM operation of multiple subqueries

Question

i have the following mysql database table designed,

ticket(id, code, cust_name);
passenger(id, ticket_id, name, age, gender, fare);
service(id, passenger_id, item, cost);

A ticket can have many passenger and each passenger can have multiple services purchased. What I want is to get the grand total of each ticket cost.

I have tried the following sql,

SELECT 
    SUM(fare) as total_fare,
    (SELECT SUM(cost) as total_cost FROM services WHERE passenger.id = services.passenger_id) as total_service_cost
FROM
    ticket
JOIN passenger ON passenger.ticket_id = ticket.id

Though, the result gets the total of passenger fare as total_fare but for the service cost, it sums and returns the first passenger's total service cost only.

I thinks i need some more nesting of queries, need help and if possible please how can i get the result as grand total summing up both passenger fare and service cost total.

OK here is the sql insert statment to clarify,

---

INSERT INTO `ticket` (`id`, `code`, `cust_name`) VALUES
(1, 'TK01', 'Dipendra Gurung');

INSERT INTO `passenger` (`id`, `ticket_id`, `name`, `age`, `gender`, `fare`) VALUES
(1, 1, 'John', '28', 'M', 120),
(2, 1, 'Kelly', '25', 'F', 120);

INSERT INTO `services` (`id`, `passenger_id`, `item`, `cost`) VALUES
(1, 1, 'S1', 30),
(2, 1, 'S2', 50),
(3, 2, 'S3', 50);

I want to get the total cost of the ticket 'TK01' (including total fare and services total). The sql must return total fare as 120+120 = 240 and total services as 30+50+50 = 130.

Thanks! :)

Answer 1

First of all in your current table schema you have no way to distinguish between services that have been sold to the same passenger in different tickets. Therefore you have no way to correctly calculate total_cost per ticket. You have to have ticket_id in your service table.

Now, if you were to have a ticket_id in service table then a solution with a correlated subqueries might look like

SELECT t.*,
       (SELECT SUM(fare) 
          FROM passenger
         WHERE ticket_id = t.id) total_fare,
       (SELECT SUM(cost) 
          FROM service
         WHERE ticket_id = t.id) total_cost
  FROM ticket t

or with JOINs

SELECT t.id, 
       p.fare total_fare,
       s.cost total_cost
FROM ticket t LEFT JOIN 
(
  SELECT ticket_id, SUM(fare) fare
    FROM passenger
   GROUP BY ticket_id
) p 
  ON t.id = p.ticket_id LEFT JOIN 
(
  SELECT ticket_id, SUM(cost) cost
    FROM service
   GROUP BY ticket_id
) s
  ON t.id = s.ticket_id

Note: Both queries take care of the fact that passenger can have multiple services per ticket or no services at all.

---

Now with your current schema

SELECT t.*,
       (SELECT SUM(fare) 
          FROM passenger
         WHERE ticket_id = t.id) total_fare,
       (SELECT SUM(cost) 
          FROM service s JOIN passenger p
            ON s.passenger_id = p.id
         WHERE p.ticket_id = t.id) total_cost
  FROM ticket t

and

SELECT t.id, 
       p.fare total_fare,
       s.cost total_cost
FROM ticket t LEFT JOIN 
(
  SELECT ticket_id, SUM(fare) fare
    FROM passenger
   GROUP BY ticket_id
) p 
  ON t.id = p.ticket_id LEFT JOIN 
(
  SELECT p.ticket_id, SUM(cost) cost
    FROM service s  JOIN passenger p
      ON s.passenger_id = p.id
   GROUP BY p.ticket_id
) s
  ON t.id = s.ticket_id

---

Just to get a grand total per ticket

SELECT t.*,
       (SELECT SUM(fare) 
          FROM passenger
         WHERE ticket_id = t.id) +
       (SELECT SUM(cost) 
          FROM service s JOIN passenger p
            ON s.passenger_id = p.id
         WHERE p.ticket_id = t.id) grand_total
  FROM ticket t

or

SELECT t.id, 
       p.fare + s.cost grand_total
FROM ticket t LEFT JOIN 
(
  SELECT ticket_id, SUM(fare) fare
    FROM passenger
   GROUP BY ticket_id
) p 
  ON t.id = p.ticket_id LEFT JOIN 
(
  SELECT p.ticket_id, SUM(cost) cost
    FROM service s  JOIN passenger p
      ON s.passenger_id = p.id
   GROUP BY p.ticket_id
) s
  ON t.id = s.ticket_id