SQL查詢左連接表

Question

我確定昨天可以實施，如果其他表沒有價值，請使用LEFT JOIN 。
我現在的問題是，如果不確定哪個表具有價值，哪個表具有價值，該怎么辦。

我已經試過這個查詢。

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

但是不幸的是，如果我的記錄表為空，它將不會顯示任何內容。

我想達到的目標是
如果table_record是空的， tbl_attendance不是，它會顯示在記錄tbl_attendance ，
如果table_attendance是空的， table_record不是，它會顯示在table_record記錄。

Answer 1

將LEFT JOIN更改為FULL OUTER JOIN，修改WHERE子句，讓我們知道它的運行方式。

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; 

這是MySql的草稿版本，其中不支持FULL OUTER JOIN。 您可以在RIGHT JOIN部分中將所有record。*字段替換為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

只需將“左聯接”更改為“完全外聯接”

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