Nested case conditionals in PostgreSQL, syntax error? - sql

Question

I use PostgreSQL and I would like to combine these two case conditions, but when I uncomment the code, I get a syntax error. This is not a difficult instruction. I want to divide or multiply the obtained number depending on the condition. How can I enter it so that the code is compiled?
SELECT
SUM(
CASE
WHEN transactions.recipient_account_bill_id = recipientBill.id AND recipientBill.user_id = 2
THEN 1
ELSE -1
END * transactions.amount_money
/* CASE
WHEN senderBillCurrency.id = recipientBillCurrency.id
THEN NULL
ELSE
CASE
WHEN recipientBillCurrency.base = true
THEN /
ELSE *
END senderBillCurrency.current_exchange_rate
END */
) as TOTAL
FROM transactions
LEFT JOIN bills AS senderBill ON senderBill.id = transactions.sender_account_bill_id
LEFT JOIN bills AS recipientBill ON recipientBill.id = transactions.recipient_account_bill_id
LEFT JOIN currency as senderBillCurrency ON senderBillCurrency.id = senderBill.currency_id
LEFT JOIN currency as recipientBillCurrency ON recipientBillCurrency.id = recipientBill.currency_id
WHERE 2 IN (senderBill.id, recipientBill.id)

Answer 1

You cannot create dynamic SQL expressions like that. A CASE expression cannot return an operator as if SQL was some sort of macro language. You can only return an expression.
You already used the following approach using 1 and -1 with your multiplication. Why not also use it with N and 1/N:
<some expression> * CASE WHEN condition THEN 1 / N ELSE N END
Or in your case:
<some expression> * CASE
WHEN senderBillCurrency.id = recipientBillCurrency.id THEN 1
WHEN recipientBillCurrency.base THEN 1 / senderBillCurrency.current_exchange_rate
ELSE senderBillCurrency.current_exchange_rate
END
Notice, you can put several WHEN clauses in a CASE expression. No need to nest them