Question

I am really having a headache since the other day on how to fix this mysql statement to get my desired result. I also want to inform that I am new to mysql and prorgramming.

I have 4 tables CUSTOMER, CUSTOMER_ACCT_SETTING, DEBT, and PAYMENT.

Here are the 4 tables with their record so you can relate.

CUSTOMER

CUSTOMER_ACCT_SETTING

DEBT

PAYMENT

When I run this mysql statement:

SELECT C.CUSTOMER_ID, C.NAME, C.ADDRESS, C.CONTACT_NUMBER,
    SUM(((CAS.INTEREST_RATE / 100) * D.AMOUNT) + D.AMOUNT) - COALESCE(SUM(P.AMOUNT), 0) AS CURRENT_BALANCE
FROM CUSTOMER C
INNER JOIN CUSTOMER_ACCT_SETTING CAS ON (C.CUSTOMER_ID = CAS.CUSTOMER_ID)
LEFT JOIN DEBT D ON (C.CUSTOMER_ID = D.CUSTOMER_ID)
LEFT JOIN PAYMENT P ON C.CUSTOMER_ID = P.CUSTOMER_ID
GROUP BY (C.CUSTOMER_ID)
ORDER BY C.NAME

The result is below:

PS: The result is ordered by name.

My question is:

1.) Why did I get a negative result on the CURRENT_BALANCE column in the first row? I am expecting the result to be around 16374.528.

My desired result is like this:

Answer 1

You are projecting your payments through all your debts by doing a join with both tables at the same time. So you essentially get 5 applications of your payment on customer 4 and zero applications on all the other customers. (so NULL on P.AMOUNT yields X – NULL = NULL). To see this, remove the “GROUP BY” and the “SUM” and just return your amounts paid and debited. Then if you group/sum these results manually by customer, you’ll see what’s going on.

To get the results you expect, you will need to use subqueries or some other mechanism like temporary tables. Something like this:

SELECT C.CUSTOMER_ID,
  (SELECT SUM(P.AMOUNT) FROM PAYMENT P 
   WHERE P.CUSTOMER_ID = C.CUSTOMER_ID) AS TOTAL_PAID_BY_CUSTOMER 
FROM CUSTOMER C