Question

I have already a query with multiple JOINs, simple list of reservations

SELECT reservation.reservation_id, customer.customer_id, customer.name, count(ordered_services.reservation_id) AS num_of_ordered_services
FROM reservations
JOIN customers ON reservations.customer_id = customer.customer_id
LEFT JOIN ordered_services ON reservations.reservation_id = ordered_services.reservation_id
GROUP BY reservation.reservation_id, customer.customer_id, customer.name
ORDER BY reservation.reservation_id

which outputs something like

reservation_id | customer_id | name | num_of_ordered_services
     1         |  1909091202 | John |          2 
     2         |  2512541508 | Jane |          3 

I would like to add another column with information about payment, but simple JOIN, LEFT JOIN interferes with existing count() column. Like

SELECT reservation.reservation_id, count(payments.reservation_id) AS num_of_payments
FROM reservations
LEFT JOIN payments ON reservations.reservation_id = payments.reservation_id
GROUP BY reservation.reservation_id
ORDER BY reservation.reservation_id

reservation_id | num_of_payments
     1         |       0 
     2         |       2

but in both a single result. How to achieve this?

PS: num_of_payments is not necessary, I only need to know if the payment for certain reservation exists or not (1, 0).
Thank you

---

tbl structure, nothing special:
reservations

reservation_id | customer_id |        added
     1         |  1909091202 | 2011-11-04 02:37:28
     2         |  2512541508 | 2011-11-04 14:27:01

customers

customer_id | name | personal information columns ...
 1909091202 | John |       |      | 
 2512541508 | Jane |       |      |  
    ...     | ...  |       |      |

payments

payment_id | reservation_id | customer_id | total |        added
     1     |        2       |  1909091202 |   199 | 2011-11-04 02:37:28
     2     |        2       |  2512541508 |    50 | 2011-11-04 14:27:01

Answer 1

You could use a subselect for the additional field.

SELECT reservation.reservation_id, customer.customer_id, customer.name, 
count(ordered_services.reservation_id) AS num_of_ordered_services,
(SELECT count(*) FROM payments WHERE reservation.reservation_id=payments.reservation_id) AS num_of_payments
FROM reservations
JOIN customers ON reservations.customer_id = customer.customer_id
LEFT JOIN ordered_services ON reservations.reservation_id = ordered_services.reservation_id
GROUP BY reservation.reservation_id, customer.customer_id, customer.name
ORDER BY reservation.reservation_id