mysql选择有两行的记录

Question

如果我有一个student_table如下：

original_student            other_student
10123                       20234    
10234                       20456 

我还有其他包含学生班的表

Student_code                Class
10123                       class_001
20234                       class_002     
10234                       class_003
20456                       class_004

我的问题是关于student_table，如何找出original_student和other_student的行的总计数大于或等于2，我需要通过组合original_student和other_student进行计数。

谢谢

Answer 1

select s.original_student, s.other_student, count(*) 
from student_table s      
inner join student_class c1 on c1.Student_code = s.original_student
inner join student_class c2 on c2.Student_code = s.other_student
group by s.original_student, s.other_student
having count(*) >= 2

Answer 2

您可以使用IN(...)加入条件通过单个联接执行此操作：

select s.original_student, s.other_student, count(distinct sc.class) 
from student_table s      
join student_class sc on sc.Student_code in (s.original_student, s.other_student)
group by s.original_student, s.other_student
having count(distinct sc.class) > 1

distinct用于count()，以防每个学生与同一个类代码相关联，这可能不应该算作两个类，而只是一个类。< / p>