How do I update a column based on the previous value? - mysql

Question

I have read a lot of answers here but I couldn't adapt to my needs.
I have this table below where I would like to update the BALANCE column:
balance = old.balance + new.amount
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
| ID | TRANSACTION_ID | BANK_ID | ACCOUNT_ID | CUSTOMER_ID | CREATED | DESCRIPTION | AMOUNT | CURRENCY | BALANCE |
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
| 1 | T1 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | 100.00 | GBP | NULL |
| 2 | T2 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | 125.00 | GBP | NULL |
| 3 | T3 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | -73.00 | GBP | NULL |
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
This is the result I would like is shown below:
I got it executing:
SET #balance:=0;
UPDATE TRANSACTIONS SET BALANCE = (#balance := #balance + AMOUNT) WHERE ID > 0;
There is no way to fire the statement above after a new column inserted?
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
| ID | TRANSACTION_ID | BANK_ID | ACCOUNT_ID | CUSTOMER_ID | CREATED | DESCRIPTION | AMOUNT | CURRENCY | BALANCE |
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
| 1 | T1 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | 100.00 | GBP | 100.00 |
| 2 | T2 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | 125.00 | GBP | 225.00 |
| 3 | T3 | 2 | 2 | 1 | 2018-04-22 00:00:00 | TRANSACTION TEST | -73.00 | GBP | 152.00 |
+----+----------------+---------+------------+-------------+---------------------+------------------+--------+----------+---------+
I tried using trigger:
DELIMITER $$
CREATE TRIGGER updateBalance AFTER INSERT ON TRANSACTIONS
FOR EACH ROW
BEGIN
SET NEW.BALANCE = BALANCE + NEW.AMOUNT;
END $$
DELIMITER ;
And I got the error:
Error Code: 1362. Updating of NEW row is not allowed in after trigger
I am new in SQL and MySQL and I believe this is a common task for advanced users.

Answer 1

Values that can be calculated from other (materialized) values shouldn't be materialized as this can lead to inconsistencies.
Remove the column all together.
ALTER TABLE transactions
DROP balance;
And create a view instead:
CREATE VIEW transactions_with_balance
AS
SELECT t1.*,
(SELECT sum(t2.amount)
FROM transactions t2
WHERE t2.bank_id = t1.bank_id
AND t2.account_id = t1.account_id
AND t2.id <= t1.id) balance
FROM transactions t1;
db<>fiddle
If you're using MySQL version 8 or higher you can also replace the subquery by the windowed version of sum()
CREATE VIEW transactions_with_balance
AS
SELECT t1.*,
sum(amount) OVER (PARTITION BY t1.bank_id,
t1.account_id
ORDER BY t1.id) balance
FROM transactions t1;
db<>fiddle
The column customer_id also seems misplaced in the table as I suppose there is an account table where the customer that account belongs to is stored in a foreign key to the customer table. So you can get the customer via the accoount_id.

Answer 2

You can do this with a BEFORE INSERT trigger, summing all the transaction amounts for the given CUSTOMER_ID and adding the new AMOUNT value to get the balance:
DELIMITER $$
CREATE TRIGGER updateBalance BEFORE INSERT ON transactions
FOR EACH ROW
BEGIN
SET NEW.BALANCE = NEW.AMOUNT +
COALESCE((SELECT SUM(AMOUNT)
FROM transactions
WHERE CUSTOMER_ID = NEW.CUSTOMER_ID), 0);
END $$
DELIMITER ;
Demo on dbfiddle
Note you may want to further qualify the sum with
AND BANK_ID = NEW.BANK_ID
and/or
AND ACCOUNT_ID = NEW.ACCOUNT_ID
as necessary to distinguish exactly which records to read the previous transactions from.