Fourth Normal Form
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Boyce-Codd Normal Form(BCNF)
A table is in fourth normal form (4NF) if and only if it is in BCNF and contains no more than one multi-valued dependency.
- Anomalies can occur in relations in BCNF if there is more than one multi-valued dependency.
- If A--->B and A--->C but B and C are unrelated, ie A--->(B,C) is false, then we have more than one multi-valued dependency.
- A relation is in 4NF when it is in BCNF and has no more than one multi-valued dependency.
Example to understand 4NF:-
Take the following table structure as an example:
info(employee#, skills, hobbies)
Take the following table:
| employee# | skills | hobbies |
|---|---|---|
1 | Programming | Golf |
1 | Programming | Bowling |
1 | Analysis | Golf |
1 | Analysis | Bowling |
2 | Analysis | Golf |
2 | Analysis | Gardening |
2 | Management | Golf |
2 | Management | Gardening |
This table is difficult to maintain since adding a new hobby requires multiple new rows corresponding to each skill. This problem is created by the pair of multi-valued dependencies EMPLOYEE#--->SKILLS and EMPLOYEE#--->HOBBIES. A much better alternative would be to decompose INFO into two relations:
skills(employee#, skill)
| employee# | skills |
|---|---|
1 | Programming |
1 | Analysis |
2 | Analysis |
2 | Management |
hobbies(employee#, hobby)
| employee# | hobbies |
|---|---|
1 | Golf |
1 | Bowling |
2 | Golf |
2 | Gardening |
