3 个表的 SQL 查询（或联接）

Question

第一次在 Stack Overflow 上提出问题...令人惊叹的资源，但作为 SQL 新手，只有一件事让我非常困惑。

我有三个表，我想获取与 Bob 的学生相关联的所有导师的姓名。

表 1：教师

================
ID     Name
================
1     Bob

表 2：学生

===================================
STUDENT_ID     Name     TEACHER_ID
===================================
1              Jayne    1
2              Billy    5
3              Mark     2

表 3：MENTOR_RELATIONSHIPS

==============================
ID     STUDENT_ID    MENTOR_ID
==============================
1      1             3
2      2             2
3      3             3

表 4：导师

=====================
MENTOR_ID     Name  
=====================
1            Sally
2            Gillian
3            Sean

我想运行一个查询来查找 Bob 学生的所有导师。所以所有学生的导师TEACHER_ID = 1

在这种情况下，肖恩将是结果。

我知道这与联接有关，或者我可以使用普通查询找到它吗？？

非常感谢任何帮助！非常感谢...

Answer 1

这应该做的工作

select distinct m.name from students s
inner join mentor_ralationships mr on mr.student_id=s.student_id
inner join mentors m on m.mentoir_id=mr.mentor_id
where s.teacher_id=1;

Answer 2

没有连接（不是首选）

SELECT mentors.name FROM mentors 
WHERE mentors.id 
  IN (SELECT MENTOR_RELATIONSHIPS.mentor FROM  MENTOR_RELATIONSHIPS 
  WHERE  MENTOR_RELATIONSHIPS.student 
    IN (SELECT students.id FROM students WHERE students.teacher 
        = (SELECT teachers.id FROM teachers WHERE teachers.name = 'bob')));

Answer 3

这可能对您有所帮助，因为当老师分配到特定主题时，我必须从三个表 AssignedSubject, Section and SchoolClass 中检索数据，然后我必须找出他的班级和部分详细信息，包括我这样做的 subjectid

select a.StaffID, a.SubjectID, s.ID as SectionId, s.Name as SectionName, S.Remarks as SectionRemarks, sc.ID as ClassId, sc.Name as ClassName, sc.Remarks as ClassRemarks from AssignedSubject a
     inner join Section s on a.SectionId=s.ID 
     inner join SchoolClass sc on sc.ID=s.ClassId  where a.StaffID=3068

Answer 4

您可以尝试以下方法：

SELECT DISTINCT m.name
FROM students s INNER JOIN TEACHERS t ON t.ID = a.TEACHER_ID
  INNER JOIN MENTOR_RELATIONSHIPS mr ON mr.Student_id = s.Student_id
  INNER JOIN Mentors m ON mr.MENTOR_ID = s.MENTOR_ID
WHERE s.teacher_id = 1   WHERE t.Name = 'Bob';

Answer 5

SELECT TEACHER.NAME, STUDENTS.NAME AS STUDENT NAME, MENTORS.NAME AS MENTOR 
FROM TEACHERS JOIN STUDENTS ON TEACHERS.ID = STUDENTS.TEACHER_ID 
              JOIN MENTOR_RELATIONSHIPS ON STUDENTS.STUDENT_ID = 
                   MENTOR_RELATIONSHIPS.STUDENT_ID 
              JOIN MENTORS ON MENTOR_RELATIONSHIPS.MENTOR_ID = MENTORS.MENTOR_ID 
WHERE TEACHER.NAME = 'Bob' ;