3个表的SQL查询（或连接）

first time asking a question on Stack Overflow... Amazing resource, but there's just one thing that's really baffling me as a newcomer to SQL.

第一次在Stack Overflow上问一个问题......令人惊讶的资源,但只有一件事让我感到困惑的是作为SQL的新手。

I have three tables and I would like to obtain the names of all the Mentors who are linked to Bob's students.

我有三张桌子,我想获得与Bob的学生有联系的所有导师的名字。

Table 1: TEACHERS

表1:教师

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

Table 2: STUDENTS

表2:学生

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

Table 3: MENTOR_RELATIONSHIPS

表3:MENTOR_RELATIONSHIPS

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

Table 4: MENTORS

表4:导师

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

I would like to run a query to find all of the mentors of Bob's students. So the mentors for all students with TEACHER_ID = 1

我想运行一个查询来查找Bob的学生的所有导师。因此,所有学生的导师都是TEACHER_ID = 1

In this case Sean would be the result.

在这种情况下,肖恩就是结果。

I know that it is something to do with Joins, or could I find this using a normal query??

我知道它与Joins有关,或者我可以使用普通查询找到它吗?

Any help is much appreciated! Many thanks...

任何帮助深表感谢!非常感谢...

4 个解决方案

#1

this should do the work

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;

#2

Without joins (not preferred)

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')));

#3

-1

You could try the following:

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';

#4

-1

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' ;

#1