Left Join With Null value Table - php

Question

I was using this query to connect my student table and attendance table,
My Problem is, sometimes, attendance table has no value.
It's not returning any value.
<?php
if($_SERVER['REQUEST_METHOD']=="POST"){
include('include/connection.php');
showData();
}
function showData(){
global $connect;
$teacher_id = $_POST['teacher_id'];
$subject_id = $_POST['subject_id'];
$date = $_POST['date'];
$query ="
SELECT s.student_name
, s.student_number
, s.student_section
, s.subject_id
, s.fingerprint_id
, s.teacher_id
, a.status
FROM tbl_student s
LEFT
JOIN tbl_attendance a
on s.subject_id=a.subject_id
WHERE s.subject_id = '$subject_id'
and a.date='$date'
and s.teacher_id = '$teacher_id';";
$result =mysqli_query($connect,$query);
$number_of_rows = mysqli_num_rows($result);
$temp_array=array();
if($number_of_rows>0){
while($row=mysqli_fetch_assoc($result)){
$temp_array[]=$row;
}
}
header('Content-Type: application/json');
echo json_encode(array("student"=>$temp_array));
mysqli_close($connect);
}
?>
What I want to achive is even if attendance table has no value,
I can still see the student fields.
Is it even possible with SQL query? Thanks

Answer 1

You have to move the fields of table attendance from where to the on condition:
$query ="SELECT student.student_name,student.student_number,student.student_section,student.subject_id,student.fingerprint_id,student.teacher_id,attendance.status
FROM tbl_student student
LEFT JOIN tbl_attendance attendance on student.subject_id=attendance.subject_id and attendance.date='$date'
WHERE student.subject_id='$subject_id' and student.teacher_id='$teacher_id';";
Because first the join Statement will be executed and then the where, if you access the table tbl_attendance in where ans all the columns are null, they will filtered out.
Hint: read about prepared Statements to provide SQL-injection

Answer 2

SELECT student.student_name,student.student_number,student.student_section,student.subject_id,student.fingerprint_id,student.teacher_id,attendance.status
FROM tbl_student student
LEFT JOIN tbl_attendance attendance on student.subject_id=attendance.subject_id and attendance.date='$date'
WHERE student.subject_id='$subject_id' and student.teacher_id='$teacher_id';
Try above code.Hope this will helps.
As you had made condition on student table using attendance.date='$date' on WHERE clause it exclude that record which are not satisfy this condition.
So instead of where i had put that condition through ON clause on LEFT JOIN.
This will achieve your goal.