Consult the above example tables when looking at this example.

SELECT m.mgr_id , m.first_name , m.last_name FROM managers m INNER JOIN employees e ON e.mgr_id = m.mgr_id WHERE e.emp_id = 'E03' ;

MGR_ID | FIRST_NAME | LAST_NAME | — | — | — | M03 | Barrel | Jones |

As this is the inverse of the above example, we know that for every employee we query for, we will only ever see one corresponding manager.