带空值的左连接表

Question

我正在使用此查询来连接学生表和出勤表，
我的问题是，有时候出勤表没有价值。
它没有返回任何值。

<?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);
}


?>

我想要达到的是即使考勤表没有价值，
我仍然可以看到学生领域。
SQL查询甚至有可能吗？ 谢谢

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';

试试上面的代码，希望这会有所帮助。 由于您已经在WHERE子句中使用attendance.date='$date'在学生表上设置了条件，因此排除了不满足此条件的记录。 因此，而不是我通过LEFT JOIN ON子句将该条件放在何处。 这将实现您的目标。

Answer 2

您必须将表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';";

因为首先将执行join语句，然后将执行where语句，因此，如果在其中ans所有tbl_attendance null的位置访问表tbl_attendance ，则它们将被过滤掉。

提示：了解准备好的语句以提供SQL注入