3657. Find Loyal Customers

Table: customer_transactions +------------------+---------+ | Column Name | Type | +------------------+---------+ | transaction_id | int | | customer_id | int | | transaction_date | date | | amount | decimal | | transaction_type | varchar | +------------------+---------+ transaction_id is the unique identifier for this table. transaction_type can be either 'purchase' or 'refund'. Write a solutio