连接四个不同的表

Question

I've got a problem on displaying data from 4 tables, I want to display students names,his/her department,a course in that department, and the marks the student got in that course. My tables are student, department, courses and marks.

Student :

student_ID
student_Fname
student_Lname

Department :

dep_ID
dep_Name

Courses :

course_ID
course_Name
dep_ID

Marks :

marks_ID
student_ID
dep_ID
course_ID
score

I tried to display the data by linking student and department table to get student names and their department,the i joined department and courses to retrieve the courses included in that department but when i tried to join the student and marks table it failed.

This is what the query returned,it returned one student and all the courses in the sports table,which i didn't even assign marks to. and it didn't display the second student's marks

student_Fname | dep_Name  | course_Name    | score
--------------+-----------+----------------+------
Berhan        | sports    | football       |    25
Berhan        | sports    | baseball       |    25
Berhan        | sports    | basketball     |    25
Berhan        | sports    | tennis         |    25

This is the data i inserted in marks table.

marks_ID | student_ID | dep_ID | course_ID | score
---------+------------+--------+-----------+------    
    1    |     1      |   3    |    7      | 25
    2    |     2      |   3    |    8      | 37

I tried this query:

SELECT 
    s.student_Fname,
    d.dep_Name,
    c.course_ID,
    m.score 
FROM 
    student s 
JOIN 
    department d ON s.dep_ID = d.dep_ID 
JOIN 
    courses c ON c.dep_ID = d.dep_ID 
JOIN 
    marks m ON m.student_ID = s.student_ID

As I only have 2 students in marks table, I need a result like this.

Berhan       | sports   | football       | 25
Hiro         | sports   | baseball       | 37

Please I need some help..

Answer 1

If you only want to print marks in courses in the student's department, you need to add that additional criteria to one of the joins.

SELECT s.student_Fname
     , d.dep_Name
     , c.course_ID
     , m.score 
  FROM student s 
  JOIN marks m ON
    ON m.student_ID = s.student_ID
  JOIN courses c
    ON c.course_ID = m.course_ID
  JOIN department d
    ON c.dep_ID = d.dep_ID AND d.dep_ID = s.dep_ID

There's also no need to have dep_ID in the marks table. Since course has a foreign key to department , putting it in marks is redundant: the mark can't be in a different department than the one associated with the course.