SELECT c.site_id, c.tester_model
     , AVG(cpu_med_time) AS Median
  FROM (SELECT site_id, tester_model, cpu_med_time
             , @rn := CASE WHEN @site_id      = site_id
                            AND @tester_model = tester_model
                           THEN @rn + 1 ELSE 1 END AS rn
             , @site_id      := site_id
             , @tester_model := tester_model
          FROM table1 a
             , (SELECT @rn           := 0
                     , @site_id      := ''
                     , @tester_model := ''
                ) b
         ORDER BY site_id, tester_model, cpu_med_time
        ) c
 INNER JOIN
       (SELECT site_id, tester_model
             , COUNT(*) cnt
          FROM table1 a
         GROUP BY site_id, tester_model
        ) d
    ON c.site_id      = d.site_id
   AND c.tester_model = d.tester_model
   AND c.rn BETWEEN d.cnt/2 AND d.cnt/2+1
 GROUP BY c.site_id, c.tester_model
;