Fifth normal form (5NF)

• A relation is in 5NF if it is in 4NF and not contains any join dependency and joining should be lossless.
• 5NF is satisfied when all the tables are broken into as many tables as possible in order to avoid redundancy.
• 5NF is also known as Project-join normal form (PJ/NF).

Example

In the above table, John takes both Computer and Math class for Semester 1 but he doesn't take Math class for Semester 2. In this case, combination of all these fields required to identify a valid data.

Suppose we add a new Semester as Semester 3 but do not know about the subject and who will be taking that subject so we leave Lecturer and Subject as NULL. But all three columns together acts as a primary key, so we can't leave other two columns blank.

So to make the above table into 5NF, we can decompose it into three relations P1, P2 & P3:

P1

P2

P3