MySQL條件不包括一些所需的記錄

我有5個表，我想加入到MySQL中，以返回參加課程的學生列表，他們參加的課程，這些課程中的任務以及學生的成績在每個作業中獲得（如果學生尚未完成作業，則爲NULL）。MySQL條件不包括一些所需的記錄

table_students 
id, name 
1, John 
2, Jacob 
3, Jingleheimer 
4, Schmidt 

table_courses 
id, name 
20, English 
30, Math 
40, Science 

table_assignments 
id, courseid, name 
1, 20, English Assignment 1 
2, 20, English Assignment 2 
3, 20, English Assignment 3 
4, 30, Math Assignment 1 
5, 30, math Assignment 2 
6, 40, Science Assignment 1 
7, 40, Science Assignment 2 
8, 40, Science Assignment 3 
9, 40, Science Assignment 4 

table_course_enrollments 
studentid, courseid 
1, 30 
1, 40 
3, 30 
3, 20 
4, 40 

table_assignment_grades 
studentid, courseid, assignmentid, grade 
1, 30, 4, A 
1, 40, 6, C 
1, 40, 8, B 
1, 40, 9, A 
3, 30, 4, D

我要回：

Student Name,Course Name, Assignment Name, Grade 
John, Math, Math Assignment 1, A 
John, Math, Math Assignment 2, Null 
John, Science, Science Assignment 1, C 
John, Science, Science Assignment 2, Null 
John, Science, Science Assignment 3, B 
John, Science, Science Assignment 4, D 
Jingleheimer, Math, Math Assignment 1, D 
Jingleheimer, Math, Math Assignment 2, Null 
Jingleheimer, English, English Assignment 1, Null 
Jingleheimer, English, English Assignment 2, Null 
Schmidt, Science, Science Assignment 1, Null 
Schmidt, Science, Science Assignment 2, Null 
Schmidt, Science, Science Assignment 3, Null 
Schmidt, Science, Science Assignment 4, Null

（無數據雅各，因爲他沒有在任何課程就讀）

我的查詢：

SELECT table_students.name, table_courses.name, table_assignments.name, table_assignment_grades.grade 

FROM table_students AS s 
JOIN table_course_enrollments AS ce ON ce.userid=s.id 
JOIN table_courses AS c ON c.id=ce.courseid 
JOIN table_assignments AS a ON a.courseid=c.id 
LEFT JOIN table_assignment_grades AS ag ON ag.userid=u.id 

WHERE ag.assignmentid=a.id 
AND ag.courseid=c.id

結果包括如預期的學生和課程，但只有學生完成的成績和作業，而不是所有作業，c完成與否。如果我刪除WHERE子句，我會得到所有作業的列表，但是成績和作業彼此不匹配。需要將assignment_grades表連接到學生，作業和課程才能正確報告。

如何重寫這個表格，僅返回參加課程的學生，這些課程中的作業，以及每個作業的成績，即使作業尚未評分？

非常感謝任何提示！伯

來源

2013-05-28 Kimber Warden

您需要將WHERE條件移動到左加入ON條款，否則你會過濾掉所有行沒有等級顯示，而是按照您的設置將它們設置爲NULL;

SELECT s.name sname, c.name cname, a.name aname, ag.grade 
FROM table_students AS s 
JOIN table_course_enrollments AS ce ON ce.studentid=s.id 
JOIN table_courses AS c ON c.id=ce.courseid 
JOIN table_assignments AS a ON a.courseid=c.id 
LEFT JOIN table_assignment_grades AS ag 
    ON ag.studentid=s.id 
AND ag.assignmentid=a.id 
AND ag.courseid=c.id 
ORDER BY s.id,c.id

An SQLfiddle to test with。

來源

2013-05-28 20:03:59

感謝您的詳細回覆！ –

只是把標準從WHERE在過去ON應該修復它：

SELECT table_students.name, table_courses.name, table_assignments.name, 
     table_assignment_grades.grade 
FROM table_students AS s 
JOIN table_course_enrollments AS ce ON ce.userid=s.id 
JOIN table_courses AS c ON c.id=ce.courseid 
JOIN table_assignments AS a ON a.courseid=c.id 
LEFT JOIN table_assignment_grades AS ag ON ag.userid=u.id 
    AND ag.assignmentid=a.id 
    AND ag.courseid=c.id

來源

2013-05-28 20:00:57 Dukeling

MySQL條件不包括一些所需的記錄

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