Question

我试图在ruby on rails上执行以下操作，但即使我可以在mysql中得到一个很棒的答案。

我有学生和课程表。学生 - ＆gt;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')

我希望使用单个SQL JOIN语句或使用activerecord来实现相同的结果。

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);

我恐怕此刻无法测试，我怀疑这些查询可能无法帮助您。我没有使用ActiveRecord的经验。请在评论中告诉我。

Answer 2

预言：

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'")

如果其中一个关联表的列无法满足条件，我如何忽略表中的行？

2 个答案: