SQL query question

I have two tables of which name is exam_history and name_master.
Here is sample data.
<exam_history>
      patient_id              series_id                 save_datetime
--------------------------------------------------------------------------------
*          1                           1                   09/27/2010 12:08:04         
*          1                           1                   10/13/2010 08:02:15         
            1               &nbs ...