Combine orders (whose key column is cust_id) with customers (whose key column is id), keeping every row from orders even when its cust_id has no matching customers row (fill customer fields with NaN in that case). Drop the now-redundant id column from the result.

Sample data: orders:

   order_id  cust_id
0         1       10
1         2       20
2         3       99

customers:

   id   name
0  10  Alice
1  20    Bob
2  30  Carol