即使id不存在，Mysql也会使用学生姓名获取所有主题

Question

我的数据库结构如下：

学生表

      id name gender
       1  abc  male
       2  xyz  female

科目表

       id  name
        1  maths
        2  science
        3  english

studentmarks

      id st_id  sub_id marks
       1   1       1    20
       2   1       2    30
       3   2       1    40

我想要这样的结果：

      name  subjects  marks 
       abc    maths     20
       abc    science   30  
       xyz     maths    40
       abc     english  0
       xyz     science  0
       xyz      english 0

单独使用MySQL查询是否可以实现上述结果？

我尝试了这个查询：

SELECT * 
FROM students 
LEFT JOIN studentmarks 
    ON studentmarks.std_id = students.id 
LEFT JOIN subjects 
    ON subjects.id = studentmarks.sub_id

结果不是我想要的，因为我使用join因此它只会返回匹配的id。

Answer 1

这可能有用

SELECT cr.st_name, cr.sub_name, COALESCE(c.marks, 0) 
FROM (SELECT a.id   st_id, 
             a.NAME st_name, 
             b.NAME sub_name, 
             b.id   sub_id 
        FROM students a, 
             subjects b) cr 
LEFT JOIN studentmarks c
    ON cr.sub_id = c.sub_id 
    AND c.st_id = cr.st_id 

Answer 2

您可以使用以下查询

来实现结果

SELECT SM.marks, 
    S.NAME AS Student_name, 
    SBJ.NAME as Subject
FROM  studentmarks AS SM 
JOIN students AS S ON SM.st_id = S.id 
JOIN subjects AS SBJ ON SM.sub_id = SBJ.id

你也可以使用CROSS加入，我想如果你想坚持你当前的查询

Answer 3

试一下

SELECT A.name, B.name, C.marks
FROM students A, subjects B, studentmarks C
WHERE A.id = C.st_id AND B.id = C.sub_id;