SQL查询左连接表

Question

我确定昨天可以实施，如果其他表没有价值，请使用LEFT JOIN 。
我现在的问题是，如果不确定哪个表具有价值，哪个表具有价值，该怎么办。

我已经试过这个查询。

$query ="SELECT record.student_name,record.student_id,record.student_finger,record.student_section,record.activity_type,record.activity_title,record.score,record.teacher_id,record.activity_id,record.subject_id,attendance.status,attendance.date
        FROM tbl_record record
        LEFT JOIN tbl_attendance attendance on record.student_id=attendance.student_number
        WHERE record.subject_id='$subject_id' and record.teacher_id='$teacher_id' and record.student_section='$section';";  

但是不幸的是，如果我的记录表为空，它将不会显示任何内容。

我想达到的目标是
如果table_record是空的， tbl_attendance不是，它会显示在记录tbl_attendance ，
如果table_attendance是空的， table_record不是，它会显示在table_record记录。

Answer 1

将LEFT JOIN更改为FULL OUTER JOIN，修改WHERE子句，让我们知道它的运行方式。

SELECT 
    record.student_name,
    coalesce(record.student_id, attendance.student_number),
    record.student_finger,
    record.student_section,
    record.activity_type,
    record.activity_title,
    record.score,
    record.teacher_id,
    record.activity_id,
    record.subject_id,
    attendance.status,
    attendance.date
FROM tbl_record record
FULL OUTER JOIN tbl_attendance attendance on record.student_id=attendance.student_number
WHERE (record.subject_id='$subject_id' and record.teacher_id='$teacher_id' 
       and record.student_section='$section') 
   or record.subject_id is null; 

这是MySql的草稿版本，其中不支持FULL OUTER JOIN。 您可以在RIGHT JOIN部分中将所有record。*字段替换为NULL：

SELECT 
    record.student_name,
    record.student_id,
    record.student_finger,
    record.student_section,
    record.activity_type,
    record.activity_title,
    record.score,
    record.teacher_id,
    record.activity_id,
    record.subject_id,
    attendance.status,
    attendance.date
FROM tbl_record record
LEFT JOIN tbl_attendance as attendance on 
    record.student_id = attendance.student_number
WHERE (record.subject_id='$subject_id' and record.teacher_id='$teacher_id' 
       and record.student_section='$section') 
UNION
SELECT 
    record.student_name,
    attendance.student_number,
    record.student_finger,
    record.student_section,
    record.activity_type,
    record.activity_title,
    record.score,
    record.teacher_id,
    record.activity_id,
    record.subject_id,
    attendance.status,
    attendance.date
FROM tbl_record as record
RIGHT JOIN tbl_attendance as attendance on 
    record.student_id = attendance.student_number
WHERE record.subject_id is null
; 

Answer 2

只需将“左联接”更改为“完全外联接”

           SELECT * 
             FROM tbl_Students S
  FULL OUTER JOIN tbl_Attendance A
               ON S.StudentID = A.AttendanceStudentID