Question

I have three tables STUDENT, DEPARTMENT and COURSE in a University database…
STUDENT has a UID as a Primary key -> which is the UNIQUE ID of the student
DEPARTMENT has Dept_id as a Primary Key -> which is the Dept. number
COURSE has C_id as Primary Key -> which is the Course/subject Id

I need to store marks in a table by relating the primary key of STUDENT, DEPARTMENT and COURSE for each student in each course.

UID Dept_id C_id marks
1    CS     CS01  98
1    CS     CS02  96
1    ME     ME01  88
1    ME     ME02  90

The problem is if i create a table like this for marks then i feel the data operator might insert wrong combination of primary key of a student for example

UID Dept_id C_id marks
1    CS     CS01  98
1    CS     CS02  96
1    ME     CS01  88 //wrong C_id (course id) inputted by the DBA
1    ME     ME02  90

In which case how can i prevent him doing this?
Also is there any other way to store marks for each student ? I mean like :

UID Dept_id CS01 CS02
1    CS      98   96
3    CS      95   92

Answer 1

You should avoid duplicating data in your database if possible:

UID Dept_id C_id marks
1    CS     CS01  98
     ^^     ^^

You could:

• Change the course ID to a two column key (department, course number), eg (‘CS’, ’01’).

or:

• Keep the course name as it is, but put the department ID field in the course table and omit it from your marks table. If you need to calculate the total marks for a specific department you can still do this easily by adding a JOIN to your query.

Your last suggestion seems to be a bad idea. You would need a column in your table for every course and most values would be NULL.