Running balance per account

A statement shows the balance after every transaction, not just the total.

Input

transactions txn_id bigint -- unique account_id string posted_at string -- ISO-8601 UTC, 'YYYY-MM-DDTHH:MM:SSZ' amount double -- negative for a withdrawal, null if unposted

Output

Every input row, with the columns txn_id, account_id, posted_at, amount and balance — the sum of this account's amount up to and including this transaction, rounded to 2 decimal places.

Order by posted_at, breaking ties with the smaller txn_id first.

Example

txn_id	account_id	posted_at	amount	balance
1	A1	2024-01-01T00:00:00Z	100.0	100.0
2	A1	2024-01-02T00:00:00Z	50.0	150.0
3	A1	2024-01-03T00:00:00Z	-30.0	120.0

Constraints

Two transactions can share a posted_at. They get different balances, in txn_id order — the earlier one does not yet include the later.
That is also where the default window frame bites. An ordered window with no frame is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, and RANGE advances a whole group of tied rows at once.
A null amount adds nothing and does not blank the balance.
Row order is not graded. Column names and their order are.

Community-reported interview topic. Not an official company question and no affiliation is implied.