What is "fifth normal form"

n. (context databases English) A stage in the normalization of a relational database in which a database is in fourth normal form and every join dependency is implied by the candidate keys.

Fifth normal form (5NF), also known as project-join normal form (PJ/NF) is a level of database normalization designed to reduce redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships. A table is said to be in the 5NF if and only if every non-trivial join dependency in it is implied by the candidate keys.

A join dependency *{A, B, … Z} on R is implied by the candidate key(s) of R if and only if each of A, B, …, Z is a superkey for R.

The fifth normal form was first described by Ronald Fagin in his 1979 conference paper Normal forms and relational database operators.