Concentration Risk

A branch of Northline Bank carries concentration risk when too much of its activity rests on too few customers: lose one of them and the branch's numbers collapse. Risk wants each customer's slice of the branch's total money moved, expressed as a percentage.

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 two columns, customer_id and volume_pct, one row per customer with at least one movement. volume_pct is the money that customer moved, ignoring direction, as a percentage of the money moved by everyone, rounded to 1 decimal place. Largest slice first; two customers on the same percentage come back with the smaller customer_id first.

Example output — shape only, on invented customers. The slices are percentages out of 100 and add up to the whole bank; two customers tie here, so the smaller id goes first: