Write a SQL query that returns the first and last purchase dates for each customer-product pair, only including pairs with at least two completed orders.

Create a CTE named completed_orders that joins orders and order_items, filtering status = 'completed'. In the main query, group by customer_id and product_id, selecting MIN(order_date) as first_date, MAX(order_date) as last_date, and COUNT(DISTINCT order_id) as order_cnt. Add a HAVING clause to keep only rows where order_cnt >= 2. This query uses a single aggregation pass and is efficient for large tables.