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.

1. Anomalies can occur in relations in BCNF if there is more than one multi-valued dependency.


2. 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.


3. 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:

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)

hobbies(employee#, hobby)