mysql - 加入这两个特定的表

Question

我今天遇到了一个有趣的问题（或者在我看来是这样），我认为分享它会很有成效。

我有两个具有以下结构的数据库表：

表 1：mod_class

class_id
CSD2311 
CSD2314
CSD12

表 2：m_classes_enrolled_in

student_id   class_id
5            CSD2311
5            CSD2314

因此，第一个表包含所有可能的 class_id，第二个表包含 class_id 和该班级的学生。我想创建一个学生未注册的所有课程的列表。到目前为止，左连接看起来非常好，但似乎不可能以这种方式显示它：

SELECT mod_enrolled_in.class_id 
FROM mod_enrolled_in 
WHERE student_id <> 5 
INNER JOIN mod_class 
ON mod_enrolled_in.class_id = mod_class.class_id

任何帮助将不胜感激，-D

Answer 1

你可以通过几种方式做到这一点

1)

Select class_id from mod_class
EXCEPT
    Select class_id From m_classes_enrolled_in WHERE student_id = 5

2)

Select class_id from mod_class
Where class_id NOT IN 
      (Select class_id From m_classes_enrolled_in WHERE student_id = 5)

3)

Select class_id from mod_class m
LEFT JOIN 
    (Select class_id From m_classes_enrolled_in WHERE student_id = 5) cs
    ON m.class_id = cs.class_id
WHERE cs.class_id IS NULL

Answer 2

这就是你要找的。

SELECT mod_enrolled_in.class_id 
FROM mod_enrolled_in 
LEFT JOIN mod_class 
ON mod_enrolled_in.class_id = mod_class.class_id
WHERE class_id != 5 
AND mod_class.class_id is NULL

Answer 3

你不想LEFT JOIN mod_class这样你可以做一个反加入吗？

SELECT mod_class.class_id
FROM mod_class
LEFT JOIN mod_enrolled_in ON (mod_class.class_id = mod_enrolled_in.class_id
AND mod_enrolled_in.student_id = 5)
WHERE mod_enrolled_in.class_id IS NULL

Answer 4

尝试这个：

SELECT c.class_id
FROM mod_class c

LEFT OUTER JOIN mod_enrolled_in e
ON e.class_id = c.class_id
AND e.student_id = ?

WHERE e.student_id IS NULL;

Answer 5

用这个：-

Select class_id from mod_class
Where class_id NOT IN (Select class_id From m_classes_enrolled_in)

看到这个SQL FIDDLE