Whose Balance Grew the Least?

Relationship managers at Northline Bank make a handful of check-in calls each cycle, and the one they most want to make is to the customer whose balance moved least in their favour — often the first sign that the money is going somewhere else.

The ledger makes this a small piece of arithmetic rather than a lookup. There is no balance column: there are only individual movements, and the sign is already carried on the amount, so money leaving is stored as a negative number. A customer's position for the cycle is what all their movements come to when taken together, and no row holds that figure.

transactions — one row per movement of money. amount is in dollars and is negative when money leaves the account. type says what kind of movement it was.

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

Task: Write a query returning a single row with two columns, customer_id and net_change, for the customer whose movements come to the smallest total for the cycle. Every movement counts, of every type, and the signs in the data are already correct — nothing needs flipping. Exactly one row comes back. No two Northline customers tie on this data, so there is no tie to settle.

Example output — shape only, on an invented customer; a cycle that ended down on the month comes back negative.