text: # Time:  O(n)
  # Space: O(n)
  
  SELECT d.driver_id,
         COUNT(r2.passenger_id) AS cnt
  FROM   (SELECT DISTINCT r1.driver_id FROM Rides r1) d
         LEFT JOIN Rides r2 ON d.driver_id = r2.passenger_id
  GROUP BY d.driver_id
  ORDER BY NULL;
  