The Running Balance

A Northline Bank customer has queried their statement, and support needs to reconstruct what the balance read after each of customer 1's movements — the running balance, meaning the total of everything up to and including that line. Only customer 1's rows belong on the statement.

transactions — one row per movement of money. customer_id points at a row in customers; type is one of deposit, withdrawal or transfer; txn_date is the day it settled. The ledger is signed: money arriving in an account is stored as a positive amount, and money leaving it as a negative one, so a 600-dollar withdrawal is recorded as -600.

The table already exists in the database — there is nothing to create or load.

Task: Write a query that returns three columns — txn_date, type and balance — one row per movement belonging to customer 1, in date order from earliest to latest, with id breaking any same-day tie. balance is the total of that customer's amounts up to and including the row it sits on, rounded to 2 decimal places, starting from zero before the first movement.

Example output — shape only, on invented movements. Each balance carries the ones above it along with the row it sits on, so the column rises and falls as the customer pays in and takes out: