数据库设计（七）第三范式(3NF)

In our last tutorial, we learned about the second normal form and even normalized our Score table into the 2nd Normal Form.

So let's use the same example, where we have 3 tables, Student, Subject and Score.

Student Table

Subject Table

Score Table

In the Score table, we need to store some more information, which is the exam name and total marks, so let's add 2 more columns to the Score table.

Requirements for Third Normal Form

For a table to be in the third normal form,

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

What is 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

The new Exam table

Advantage of removing Transtive Dependency

The advantage of removing transtive dependency is,

Amount of data duplication is reduced.
Data integrity achieved.

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