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Normal Forms NF3

Requirements for Third Normal Form

For a table to be in the third normal form,

  1. It should be in the Second Normal form.
  2. And it should not have Transitive Dependency.

Transitive Dependency

With exam_name and total_marks added to our Score table, it saves more data now. Primary key for our Score table is a composite key, which means it’s made up of two attributes or columns → student_id + subject_id.

Our new column exam_name depends on both student and subject. For example, a mechanical engineering student will have Workshop exam but a computer science student won’t. And for some subjects you have Prctical exams and for some you don’t. So we can say that exam_name is dependent on both student_id and subject_id.

And what about our second new column total_marks? Does it depend on our Score table’s primary key?

Well, the column total_marks depends on exam_name as with exam type the total score changes. For example, practicals are of less marks while theory exams are of more marks.

But, exam_name is just another column in the score table. It is not a primary key or even a part of the primary key, and total_marks depends on it.

This is Transitive Dependency. When a non-prime attribute depends on other non-prime attributes rather than depending upon the prime attributes or primary key.

How to remove Transitive Dependency?

Again the solution is very simple. Take out the columns exam_name and total_marks from Score table and put them in an Exam table and use the exam_id wherever required.

Score Table: In 3rd Normal Form

score_id student_id subject_id marks exam_id
         
         
         

The new Exam table

exam_id exam_name total_marks
1 Workshop 200
2 Mains 70
3 Practicals 30

Advantage of removing Transitive Dependency

The advantage of removing transitive dependency is,

  • Amount of data duplication is reduced.
  • Data integrity achieved.

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