Question

I want to create a database schema for a system that holds customers and the charges and payments they do. What is the recommended approach so that I can later get a list of all customers with their total charges and payments using only one query?

A single query should return a result like this:

NAME  |  TotalCharges  |  TotalPayments
John          80              100
Nick         100               90

I am thinking to create two separate tables, one for the charges and one for the payments.

CUSTOMERS
-ID
-NAME

CHARGES
-ID
-CUSTOMER_ID
-AMOUNT

PAYMENTS
-ID
-CUSTOMER_ID
-AMOUNT

Another approach is to create one table for both charges and payments, using one column for each, like this:

CUSTOMERS
-ID
-NAME

CHARGES_AND_PAYMENTS
-ID
-CUSTOMER_ID
-CHARGE_AMOUNT
-PAYMENT_AMOUNT

Another approach is to use one table for both charges and payments, using a TRANSACTION_TYPE flag. 0->Payment, 1->Charge, but if I use this approach I would not be able to get total charges and payments for a customer using one query (or can I?)

CUSTOMERS
-ID
-NAME

CHARGES_AND_PAYMENTS
-ID
-CUSTOMER_ID
-TRANSACTION_TYPE  (Flag: 0=Payment, 1=Charge)
-AMOUNT

Which is the best approach?

Answer 1

Do a sum on the Charges and Payments table per Customer ID

select Customers.Name
 , Customers.id
 , ( select sum(Charges.Amount)
       from Charges
      where CustomerId = Customers.id )
 , ( select sum(Payments.Amount)
       from Payments
       where CustomerId = Customers.id )