Window function lag() in trigger uses default instead of previous value - sqlite

Question

I'm trying to create an SQLite trigger to update balance for a particular account code.
accounts table :
CREATE TABLE accounts (
year INTEGER NOT NULL,
month INTEGER NOT NULL CHECK(month BETWEEN 1 AND 12),
amount REAL NOT NULL CHECK(amount >= 0),
balance REAL,
code INTEGER NOT NULL
);
When a new row is inserted I want the balance value of the new row to reflect OLD balance + NEW amount. But this trigger does not recognize the lagging balance value and I cannot figure out why:
CREATE TRIGGER trg_accounts_balance
AFTER INSERT ON accounts
BEGIN
UPDATE accounts
SET balance = (
SELECT
lag(balance, 1, 0) OVER (
PARTITION BY code
ORDER BY month
) + NEW.amount
FROM accounts
)
WHERE rowid = NEW.ROWID;
END;
If I insert one row per month, I expect my data to look like:
year
month
amount
balance
code
2022
1
100.0
100.0
100
2022
2
9.99
109.99
100
But I get:
year
month
amount
balance
code
2022
1
100.0
100.0
100
2022
2
9.99
9.99
100
What am I doing wrong?

Answer 1

The query:
SELECT
lag(balance, 1, 0) OVER (
PARTITION BY code
ORDER BY month
)
FROM accounts
returns as many rows as there are in the table and SQLite picks the first (whichever it is) to return it as the result so that it can use it to add NEW.amount.
There is nothing that links this value to the specific row that was inserted.
Instead, use this:
CREATE TRIGGER trg_accounts_balance
AFTER INSERT ON accounts
BEGIN
UPDATE accounts
SET balance = COALESCE(
(
SELECT balance
FROM accounts
WHERE code = NEW.code
ORDER BY year DESC, month DESC
LIMIT 1, 1
), 0) + NEW.amount
WHERE rowid = NEW.ROWID;
END;
The subquery returns the previous inserted row by ordering the rows of the specific code descending and skipping the top row (which is the new row).
See the demo.