从两个表中选择具有两个外键的多对多关系表中的行

Question

我很抱歉我的标题不好，因为我在选择标题时感到困惑。但我希望描述可以解释它。尽管关系表的数据为空，但我想从具有两个表的外键的关系表中选择值，所以这是我的表

我有三张桌子：

Student
==========
id || name
===============
1  || Rooney
2  || Carrick
3  || Smalling
4  || De Gea

然后：

Item
==========
id || Title
===============
1  || Pre-Test
2  || Post-Test
3  || Final-Test

然后是多对多关系的表

Score
==========
id || student_id || item_id || Score
=====================================
1  || 1          || 1       || 100
2  || 1          || 2       || 80
3  || 2          || 1       || 90
4  || 2          || 3       || 85
5  || 3          || 2       || 80
6  || 3          || 3       || 90
7  || 4          || 1       || 95

我想得到这样的结果：

Result
==========
score_id || student_name || item_name || Score
================================================
1        || Rooney       || Pre-Test  || 100
2        || Rooney       || Post-Test || 80
NULL     || Rooney       || Final-Test|| NULL
3        || Carrick      || Pre-Test  || 90
NULL     || Carrick      || Post-Test || NULL
4        || Carrick      || Final-Test|| 85
NULL     || Smalling     || Pre-Test  || NULL
5        || Smalling     || Post-Test || 80
6        || Smalling     || Final-Test|| 90
7        || De Gea       || Pre-Test  || 95
NULL     || De Gea       || Post-Test || NULL
NULL     || De Gea       || Final-Test|| NULL 

我从另一个论坛搜索了一些关于外部连接的教程并在我的数据库中尝试了它，但查询没有显示空值。谢谢，

*)已编辑

我试过用 union 和 join 查询，但结果没有显示空值

SELECT score.id AS score_id, student.name, item.title, score.score
FROM student
RIGHT JOIN score ON student.id = score.student_id
RIGHT JOIN item ON score.item_id = item.id
UNION 
SELECT score.id AS score_id, student.name, item.title, score.score
FROM item
RIGHT JOIN score ON score.item_id = item.id
RIGHT JOIN student ON score.student_id = student.id
ORDER BY score_id
LIMIT 0 , 30

和

SELECT score.id AS score_id, student.name AS student_name, item.title AS item_title, score.score
FROM student
LEFT JOIN score ON student.id = score.student_id
LEFT JOIN item ON score.item_id = item.id
LIMIT 0 , 30

更改为右连接仍然没有显示空值，结果如下：

score_id || student_name || item_title || score
1        || Rooney       || Pre-Test   || 100
2        || Rooney       || Post-Test  || 80
3        || Carrick      || Pre-Test   || 90
4        || Carrick      || Final-Test || 85
5        || Smalling     || Post-Test  || 80
7        || Smalling     || Final-Test || 90
8        || De Gea       || Pre-Test   || 95

Answer 1

使用CROSS JOIN

SELECT 
    sc.id AS score_id, 
    s.name AS student_name, 
    i.title AS item_name, 
    sc.score 
FROM student s
CROSS JOIN item i
LEFT JOIN score sc ON sc.item_id = i.id AND sc.student_id = s.id;