Question

我必须从两个表中获取数据（PK）=主键
（FK）=外键

TABLE1- [STUDENTS]
s_id(PK)     name    other
1             a        z
2             b        z
3             c        z

TABLE2- [CLASSES]
c_id(PK)  class_name
1           5th
2           6th
3           7th

TABLE3- [STUDENT-CLASS]
id(PK)    student_id(FK)     class_id(FK)
1            1                1
2            1                2

3            2                1
4            2                2

5            3                1

6            1                3

我想向学生展示当前班级（最后指定的班级）
表关系为
当学生被录取时，它被分配到1级 1年后，在[学生 - 课程]表格中插入新记录，为每个或某个学生分配新课程我希望像这样显示

s_id       name    other     [STUDENT-CLASS].Class_id    [CLASSES].class_nam
1         a        z                  3                        7th
2         b        z                  2                        6th
3         c        z                  1                        5th

Answer 1

尝试这样的事情

Select S.studentid, s.name, s.other, c.classid, c.classname
 from 
 (Select studentid, Max(Classid) as 'currentclassid' from StudentClassTable group by studentid) A
  inner join StudentTable S on A.studentid = S.Studentid 
  inner join ClassTable C on A.CurrentClassid = C.Classid

Answer 2

以下查询将完成此任务。

SELECT student_id, name, other, b.last_class_id, c.class_name
FROM STUDENTS a
LEFT JOIN (SELECT student_id, max(class_id) As last_class_id 
           FROM student_class
           GROUP BY student_id) b ON a.student_id = b.student_id
LEFT JOIN CLASSES c ON c.class_id = b.last_class_id

Sql Query从关联表中获取数据

2 个答案: