Describe the concept of join dependency and describe how this concept relates to 5NF. Provide an example to illustrate your answer.
What will be an ideal response?
Lossless-join dependency A property of decomposition, which ensures that no spurious tuples
are generated when relations are reunited through a natural join operation.
In splitting relations by projection, we are very explicit about the method of decomposition. In
particular, we are careful to use projections that can be reversed by joining the resulting relations,
so that the original relation is reconstructed. Such a decomposition is called a lossless-join (also
called a nonloss or nonadditive) decomposition, because it preserves all the data in the original
relation and does not result in the creation of additional spurious tuples. However, there are cases
were we require to perform a lossless-join decompose of a relation into more than two relations.
These cases are the focus of the lossless-join dependency and fifth normal form (5NF).
Fifth Normal Form (5NF) A relation that has no join dependency.
Fifth normal form (5NF) (also called project-join normal form (PJNF)) specifies that a 5NF
relation has no join dependency.
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