如果表中关联表的列之一不满足条件，如何忽略表中的行？

Question

我正在尝试在 ruby​​ on Rails 中执行以下操作，但即使我可以在 mysql 中得到答案，那就太好了。

我有学生表和课程表。学生->课程是一对多的关系。

  Student                   Course
|--------------|   |--------------------------------------| 
|id| name      |   |id | course_name | grade | student_id | 
|----------    |   |--------------------------------------| 
|S1| student 1 |   |C1 | Course1     | A     | S1         |
|S2| student 2 |   |C2 | Course2     | C     | S1         |
|----------    |   |C3 | Course1     | A     | S2         |
                   |C4 | Course2     | B     | S2         |
                   |--------------------------------------|

select * from Student 
where id NOT IN (select student_id from Course where grade = 'C')

我想使用单个 SQL JOIN 语句或使用 activerecord 获得相同的结果。

Answer 1

SELECT * FROM Student s 
LEFT JOIN Course c ON s.id=c.student_id AND c.grade = 'C' 
WHERE c.student_id IS NULL;

或加入您的子查询

SELECT * FROM Student s 
LEFT JOIN (SELECT student_id FROM Course WHERE grade = 'C') c
WHERE c.student_id IS NULL;

或使用存在

SELECT * FROM Student s 
WHERE NOT EXISTS (SELECT NULL FROM Course WHERE grade = 'C' AND student_id=s.id);

恐怕我现在无法测试这一点，我怀疑这些查询可能对您没有帮助。我没有使用 ActiveRecord 的经验。请在评论中告诉我。

Answer 2

甲骨文：

select distinct s.*  from student s left join course c on c.student_id=s.id where c.grade!='c'

Student.all(:select=>"distinct student.*",:joins=>"left join course on course.student_id=student.id",:conditions=>"course.grade!='c'")