Who Uses More Than One Transaction Type?

Product at Northline Bank is trying to work out how much of the app people actually use. The internal shorthand for an engaged customer is somebody who does more than one thing with their account — money in and money out, or a payment to somebody else — rather than a customer who only ever tops the account up and leaves it there.

The ledger will not answer that directly. It has one row per movement, and a customer with six rows might have made six identical deposits or three different kinds of movement. The number the product team cares about is the variety in a customer's rows, and nothing in the table records variety.

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

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

Task: Write a query returning a single column, customer_id, holding every customer whose movements cover at least two different type values. Somebody with six movements all of the same kind does not qualify. Hand the ids back smallest first.

Example output — shape only, on invented ids.