What is 4NF and 5NF Database management system

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What is 4NF and 5NF? - Database management system

Presidency university, bangalore- Database management system

By:Aparna-Dasgupta

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4NF:

Fourth normal form (4NF) is a level of database normalization where there are no non-trivial multivalued dependencies other than a candidate key.

It builds on the first three normal forms (1NF, 2NF and 3NF) and the Boyce-Codd Normal Form (BCNF). It states that, in addition to a database meeting the requirements of BCNF, it must not contain more than one multivalued dependency.

Multivalued dependency is best illustrated using an example. In a table containing a list of three things - college courses, the lecturer in charge of each course and the recommended book for each course - these three elements (course, lecturer and book) are independent of one another. Changing the course’s recommended book, for instance, has no effect on the course itself. This is an example of multivalued dependency: An item depends on more than one value. In this example, the course depends on both lecturer and book.

Thus, 4NF states that a table should not have more than one of these dependencies. 4NF is rarely used outside of academic circles.

5NF:

Fifth normal form (5NF), also known as project-join normal form (PJ/NF) is a level of database normalization designed to reduce redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships. A table is said to be in the 5NF if and only if every non-trivial join dependency in it is implied by the candidate keys.

A join dependency *{A, B, … Z} on R is implied by the candidate key(s) of R if and only if each of A, B, …, Z is a superkey for R.


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