mysql中的可选where子句条件

Question

I am working on a project where I load a list of students depending on the date selected and the teacher chosen from a dropdown.The list contains those students data whose class teacher is the teacher selected. Now i am using the following query:

select s.studentcode,s.studentname,a.attdate,if(a.period='01' or 
a.period='AM',a.status,'') as attAM,if(a.period='PM',a.status,'') as attPM 
from tblstudent s left join tblattendance a on s.studentcode=a.studentcode 
where s.classcode='10002' and a.attdate='2014-04-11'

Now the problem with the above is query is that if i chose some date like 2014-04-15 then as attendance is not marked for this date then no record is found,but i want that the student list is always displayed and if attendance is marked for that date it should be displayed accordingly else those fields will be blank

The above screenshot is displaying the list which I want.Now the field AM should be blank if attendance for that date is not marked and if it is marked it should contain the corresponding attendance

Answer 1

Move your AND a.attdate = '2014-04-11' to join's ON() clause so if date is not present it still returns the records for s.classcode = '10002' and a.attdate column will be null in the result set,where filter is applied to whole resultset where as using additional condition in join will filter the records from right table ie tblattendance not on whole resultset

SELECT 
  s.studentcode,
  s.studentname,
  a.attdate,
  IF(
    a.period = '01' 
    OR a.period = 'AM',
    a.status,
    ''
  ) AS attAM,
  IF(a.period = 'PM', a.status, '') AS attPM 
FROM
  tblstudent s 
  LEFT JOIN tblattendance a 
    ON (s.studentcode = a.studentcode AND a.attdate = '2014-04-11')
WHERE s.classcode = '10002'