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# Fourth normal form (4NF)

• A relation will be in 4NF if it is in Boyce Codd normal form and has no multi-valued dependency.
• For a dependency A → B, if for a single value of A, multiple values of B exists, then the relation will be a multi-valued dependency.

### Example

STUDENT

STU_ID COURSE HOBBY
21 Computer Dancing
21 Math Singing
34 Chemistry Dancing
74 Biology Cricket
59 Physics Hockey

The given STUDENT table is in 3NF, but the COURSE and HOBBY are two independent entity. Hence, there is no relationship between COURSE and HOBBY.

In the STUDENT relation, a student with STU_ID, 21 contains two courses, Computer and Math and two hobbies, Dancing and Singing. So there is a Multi-valued dependency on STU_ID, which leads to unnecessary repetition of data.

So to make the above table into 4NF, we can decompose it into two tables:

STUDENT_COURSE

STU_ID COURSE
21 Computer
21 Math
34 Chemistry
74 Biology
59 Physics

STUDENT_HOBBY

STU_ID HOBBY
21 Dancing
21 Singing
34 Dancing
74 Cricket
59 Hockey

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