Avoid duplicate values using join with multiple tables in Firebird

I have 3 tables

Payments

PaymntID	Date	Amount
1	Ago.2021	500
2	Sep.2021	1200
3	Oct.2021	600

CashPayments

PaymntID	Amount
1	500
2	400
3	200

CreditCardPayments

PaymntID	Amount	CreditCard
2	450	Visa
2	350	MC
3	400	Visa

I need to get the PaymentID, date, amount payed in cash and amount payed in creditcard, but with the following query I’m getting duplicates.

Just a quick note, I can have two different credit cards used in one payment, but I can also have 2 different “cash payments” used in one payment.

Query:

select
PA.DATE AS PaymntDate,
PA.PaymntID AS PaymntID,
CP.amount AS CashAmount,
CCP.amount AS CreditAmount,

from Payments PA
    LEFT join CashPayments CP on PA.PaymntID = CP.PaymntID
    LEFT join CreditCardPayments CCP on PA.PaymntID = CCP.PaymntID

I’m getting the following result:

Result

PaymntID	PaymntDate	CashAmount	CreditAmount
1	Ago.2021	500	null
2	Sep.2021	400	450
2	Sep.2021	400	350
3	Oct.2021	200	400

So in this case, the cashpayment in paymntid 2 is duplicated. What I need is the following table:

Result

PaymntID	PaymntDate	CashAmount	CreditAmount
1	Ago.2021	500	null
2	Sep.2021	400	800
3	Oct.2021	200	400

Answer

You need to aggregate the values for each ID before joining. Probably the most succinct way is to use a CTE and then outer join each to your payments table, something like the following:

with cp as (
  select paymentId, Sum(Amount) Amount
  from CashPayments
  group by PaymentId
), ccp as (
  select paymentId, Sum(Amount) QAmount
  from CreditCardPayments
  group by PaymentId 
)
select p.PaymentId, p.Date,
  Coalesce(cp.Amount,0) CashAmount,
  Coalesce(ccp.Amount,0) CreditAmount
from Payments p
left join cp on cp.PaymentId = p.PaymentId
left join ccp on ccp.PaymentId = p.PaymentId