Combine orders and customers on their shared customer_id column, keeping only the rows where a customer_id appears in both tables (drop orders with no matching customer and customers with no matching order).

Sample data: orders:

   order_id  customer_id  amount
0         1           10     100
1         2           20     200
2         3           30     150

customers:

   customer_id   name
0           10  Alice
1           20    Bob
2           40  Carol