Database Reference
In-Depth Information
5.5.5 Fourth Normal Form
Fourth Normal Form (4NF) removes multiple sets of multivalued dependencies.
The Fourth Normal Form (4NF) applies to one-to-many and many-to-many relationships
and states that independent entities cannot be stored in the same table within those
relationships. Any multiple sets of data can be divided that are not directly related and
separate them into independent tables. Independent repeating groups should not be
combined in a single relation. A table is therefore in 4NF when it:
1. A table must be in 3NF or BCNF with 3NF.
2. Does not contain more than one multi-valued dependency
3. Multi-valued dependencies must be transformed into functional dependencies. This im-
plies that one value and not multiple values are dependent on a primary key.
Let us discus an example where an organization has completely unnormalized data as
shown in following table.
In the above figure, the multi-valued dependency is {ProductName}{Size}| {Supplier},
it means ProductName multi-determines Size and Supplier.” According to 4NF, multi-val-
ued dependencies must be divided into separate relations, as shown in following Figure
figures. A relation is in fourth normal form if it is in BCNF, and also, all the multi-valued
dependencies are also functional dependencies out of the candidate keys. A multiple valued
set is a field containing a comma-delimited list or collections of some kind. A collection
could be an array of values of the same type.
