mysql - 根据其他表选择数据

Question

我还是数据库和 mysql 的新手，我想学习如何使用JOINs。对不起，我只是不知道用文字表达这个案例。我希望大家通过查看这些数据能够理解。这是带有记录的表：

Table student 
student_id | student_name 
-------------------------
1            Ana
2            Billy
3            Connor

Table comp 
comp_id | subj_id | comp_name  
--------------------------
1         24         Run       
2         24         Swim       
3         24         Jump
4         25         Eat    

Table comp_mark 
semester | subj_id | student_id | comp_id | mark
-------------------------------------------------
1          24        1            1         7
1          24        1            2         4
1          24        1            3         6
1          24        2            1         4
1          24        2            2         8
1          24        3            1         9

我期待查询选择comp_mark表（更新）的结果是这样的：

student_name | semester | subject_id | comp_id  | mark
-------------------------------------------------------
Connor        1          24           1          9
Connor        null       24           2          null
Connor        null       24           3          null        

有人可以帮我吗？非常感谢

更新

我已经尝试过（Guillaume Poussell 的查询）并按 student_name 对其进行排序：

 SELECT s.student_name, cm.semester, c.subj_id AS subject_id, c.comp_id, cm.mark
 FROM student s
 CROSS JOIN comp c
 LEFT JOIN comp_mark cm ON s.student_id = cm.student_id
 AND cm.subj_id = c.subj_id
 AND cm.comp_id = c.comp_id
 ORDER BY s.student_name

结果：

Answer 1

试试这个：

SELECT s.student_name, cm.semester, c.subj_id AS subject_id, c.comp_id, cm.mark
FROM student s
CROSS JOIN comp c
LEFT JOIN comp_mark cm ON s.student_id = cm.student_id AND cm.subj_id = c.subj_id AND cm.comp_id = c.comp_id

Answer 2

认为您需要添加交叉连接以从 comp 中获取所有可能的值。像这样的东西： -

 SELECT s.student_name, c.subj_id, s.student_id, c.comp_id, cm.mark 
 FROM student s
 CROSS JOIN comp c
 LEFT OUTER JOIN comp_mark cm 
 ON s.student_id = cm.student_id 
 AND cm.semester = '1' 
 AND cm.subj_id = '1'