查詢與聯接

Question

我運行一個查詢：

select course.course,iars.id, 
     students.rollno, 
     students.name as name, 
     teachers.name as tname, 
     students.studentid, 
     attndata.studentid ,sum(attndata.obt) as obt 
     sum(attndata.benefits) as ben , (sum(attndata.max)) as abc 
from groups, students 
     left join iars 
      on iars.id 
     left join str 
      on str.studentid=students.studentid 
     left join course 
      on course.c_id=students.course 
     left join teachers 
      on teachers.id=iars.teacherid 
     join sgm 
      on sgm.studentid=students.studentid 
     left join attndata 
      on attndata.studentid=students.studentid and iars.id=attndata.iarsid 
     left join sps 
      on sps.studentid=students.studentid and iars.paperid=sps.paperid 
     left join semdef 
      on semdef.semesterid=str.semesterid 
     where students.course='1' 
      and students.status='regular' 
      and sps.paperid='5' 
      and iars.courseid=students.course 
      and iars.semester=str.semesterid 
      and semdef.month=9 
      and iars.paperid='5' 
      and str.semesterid='1' 
      and str.sessionid='12' 
      and groups.id=sgm.groupid 
     group by sps.studentid, 
      teachers.id, 
      semdef.month 
     order by 
      students.name 

在此查詢，每當我有左連接上semdef.id=attndata.mon，我得到結果爲零時的semdef.id=null的價值，但我希望所有的結果，無論semdef，但我想用它。因爲它應該獲取結果，如果值爲空。你能幫忙嗎？

Answer 1

這可能是因爲你的where子句是說

and semdef.month=9 

，你可能想

and (semdef.month=9 OR semdef.id IS NULL) 

或類似的東西。

Answer 2

這是因爲你的where子句有關於semdef表的語句。將這些添加到join子句中，將它們放在暗示內連接的位置。

如：

Left join semdef on xxx and semdef.id = attndata.min