MySQL Query获取类似行的总和

Question

这是我的student_attendance表：

 id_student   |  Id_subject  |  subject_type  |  subject_description  |  total  |  attendance  |date
      124       34               Practicals         PHY-I                  9         9          2014-07
      124       34               Practicals         PHY-I                  9         9          2014-08
      124       34               Theory             PHY-I                  9         9          2014-07
      124       34               Theory             PHY-I                 11        11          2014-08
      124       35               Practicals         CHEM-I                15        15          2014-07
      124       35               Practicals         CHEM-I                 9         9          2014-08
      124       35               Theory             CHEM-I                 7         9          2014-07
      124       35               Theory             CHEM-I                13        14          2014-08
      124       36               Theory             MAT-I                 18        18          2014-07    
      124       36               Theory             MAT-I                 15        15          2014-08

这是我的主题表：

id_subject  |  subject_description
       34             PHY-I
       35             CHEM-I
       36             MAT-I

其中id_subject是此表的主键。

这是我的subject_timetable表

   id_subject  |  subject_type
      34         Practicals
      34         Theory 
      35         Practicals
      35         Theory 
      36         Theory

student_attendance表和subject_timetable表没有主键。

现在，我希望每个科目（理论和实践）分别为每个科目提供两个月（即2014年7月和8月）的出勤率和总人数。

请帮助我获得合适的查询。 我曾经尝试过这个查询，但是在总结之后它会给某些科目带来错误的结果。

我的MYSQL查询：

SELECT sa.id_student, sa.id_subject, sa.subject_type,
 sub.subject_description, sum(sa.total) as sum,sum(sa.attendance) as attendance
FROM (student_attendance AS sa, subject AS sub, subject_timetable AS st)
WHERE sa.id_division =7 
  AND sa.date BETWEEN  "2014-07-01" AND  "2014-08-01" 
  AND sa.id_student = 124 
  AND sa.id_subject = st.id_subject 
  AND sa.subject_type = st.subject_type 
  AND st.id_subject = sub.id_subject 
group by sa.id_student,sa.subject_type, sa.id_subject
ORDER BY  `sa`.`id_student` ASC

我得到的结果是

 id_student   |  Id_subject  |  subject_type  |  subject_description  |  total  |  attendance
      124       34       Practicals         PHY-I             18        18       
      124       34       Theory             PHY-I             20        20      
      124       35       Practicals         CHEM-I            39        39                  
      124       35       Theory             CHEM-I            20        23      
      124       36       Theory             MAT-I             48        48     

正如您在结果表中所见，7月和8月的CHEM-I（实用）出勤率为9＆amp; 15，它的总和达到了24，但在我的结果表中它达到了39，同样的事情发生在MAT-I主题上。