MySQL add balance from previous rows - mysql

Question

I’ve tried a few things I’ve seen on here but it doesn’t work in my case, the balance on each row seems to duplicate.
Anyway I have a table that holds mortgage transactions, that table has a Column that stores an interest added value or a payment value.
So I might have:
Balance: 100,000
Interest added 100 - balance 100,100
Payment made -500 - balance 99,600
Interest added 100 - balance 99,700
Payment made -500 - balance 99,200
What I’m looking for is a query to pull all of these in date order newest first and summing the balance in a column depending on whether it has interest or payment (the one that doesn’t will be null) so at the end of the rows it will have the current liability
I can’t remember what the query I tried was but it ended up duplicating rows and the balance was weird
Sample structure and data:
CREATE TABLE account(
id int not null primary key auto_increment,
account_name varchar(50),
starting_balance float(10,6)
);
CREATE TABLE account_transaction(
id int not null primary key auto_increment,
account_id int NOT NULL,
date datetime,
interest_amount int DEFAULT null,
payment_amount float(10,6) DEFAULT NULL
);
INSERT INTO account (account_name,starting_balance) VALUES('Test Account','100000');
INSERT INTO account_transaction (account_id,date,interest_amount,payment_amount) VALUES(1,'2020-10-01 00:00:00',300,null);
INSERT INTO account_transaction (account_id,date,interest_amount,payment_amount) VALUES(1,'2020-10-01 00:00:00',null,-500);
INSERT INTO account_transaction (account_id,date,interest_amount,payment_amount) VALUES(1,'2020-11-01 00:00:00',300,null);
INSERT INTO account_transaction (account_id,date,interest_amount,payment_amount) VALUES(1,'2020-11-05 00:00:00',-500,null);
So interest will be added on to the rolling balance, and the starting balance is stored against the account - if we have to have a transaction added for this then ok. Then when a payment is added it can be either negative or positive to decrease the balance moving to each row.
So above example i'd expect to see something along the lines of:
I hope this makes it clearer

Answer 1

WITH
starting_dates AS ( SELECT id account_id, MIN(`date`) startdate
FROM account_transaction
GROUP BY id ),
combine AS ( SELECT 0 id,
starting_dates.account_id,
starting_dates.startdate `date`,
0 interest_amount,
account.starting_balance payment_amount
FROM account
JOIN starting_dates ON account.id = starting_dates.account_id
UNION ALL
SELECT id,
account_id,
`date`,
interest_amount,
payment_amount
FROM account_transaction )
SELECT DATE(`date`) `Date`,
CASE WHEN interest_amount = 0 THEN 'Balance Brought Forward'
WHEN payment_amount IS NULL THEN 'Interest Added'
WHEN interest_amount IS NULL THEN 'Payment Added'
ELSE 'Unknown transaction type'
END `Desc`,
CASE WHEN interest_amount = 0 THEN ''
ELSE COALESCE(interest_amount, 0)
END Interest,
COALESCE(payment_amount, 0) Payment,
SUM(COALESCE(payment_amount, 0) + COALESCE(interest_amount, 0))
OVER (PARTITION BY account_id ORDER BY id) Balance
FROM combine
ORDER BY id;
fiddle
PS. Source data provided (row with id=4) was altered according to desired output provided. Source structure was altered, FLOAT(10,6) which is not compatible with provided values was replaced with DECIMAL.
PPS. The presence of more than one account is allowed.