Question

My SQL query:

select 
    *, 
    (select sum(Amount) as TotalAmount 
     from Payment 
     where Customer.Id = Payment.CustomerId) 
from Customer 

The result:

Id - CustomerName - CustomerPhoneNumber - TotalAmount
1  - TestUser     - TestLastName        - 3000   
2  - TestUser2    - TestLastName2       - 2500 

---

This is working as I expected. But are there any better ways to get same result ?

Answer 1

If it’s MySQL, then you can do this…

SELECT
  Customer.*,
  SUM(Payment.Amount) AS TotalAmount
FROM
  Customer
LEFT JOIN
  Payment
    ON Payment.CustomerId = Customer.Id
GROUP BY
  Customer.id

In other varieties of SQL, you need to GROUP BY everything that is selected but not an aggregate.

SELECT
  Customer.Id,
  Customer.Name,
  Customer.PhoneNumber,
  SUM(Payment.Amount) AS TotalAmount
FROM
  Customer
LEFT JOIN
  Payment
    ON Payment.CustomerId = Customer.Id
GROUP BY
  Customer.Id,
  Customer.Name,
  Customer.PhoneNumber