Question

我正在使用此查询连接我的学生表和考勤表，
我的问题是，有时，出勤表没有价值它没有返回任何价值。

<?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);
}


?>

即使出勤表没有价值，我想要实现的目标是什么我仍然可以看到学生的领域甚至可以使用SQL查询吗？感谢

Answer 1

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';

尝试上面的代码。希望这会有所帮助。由于您使用attendance.date='$date' WHERE子句在学生表上创建条件，因此排除了不满足此条件的记录。因此，而不是通过ON上的LEFT JOIN条款将该条件置于何处。这将实现您的目标。

Answer 2

您必须将表格attendance的字段从where移至on条件：

$query ="SELECT student.student_name,student.student_number,student.student_section,student.subject_id,student.fingerprint_id,student.teacher_id,attendance.status
    FROM tbl_student student
    LEFT JOIN tbl_attendance attendance on student.subject_id=attendance.subject_id and attendance.date='$date'
    WHERE student.subject_id='$subject_id'  and student.teacher_id='$teacher_id';";

因为首先执行join语句然后执行where，如果你访问表tbl_attendance，其中所有列都为null，它们将被过滤掉。

提示：阅读准备好的语句以提供SQL注入

左连接Null值表

2 个答案: