Joe Celko's SQL for Smarties, 3rd Edition

2.8. Fifth Normal Form (5NF)

Fifth Normal Form (5NF), also called the Join-Projection Normal Form or the Projection-Join Normal Form, is based on the idea of a lossless JOIN or the lack of a join-projection anomaly. This problem occurs when you have an n-way relationship, where n > 2. A quick check for 5NF is to see if the table is in 3NF and all the candidate keys are single columns.

As an example of the problems solved by 5NF, consider a table of house notes that records the buyer, the seller, and the lender:

HouseNotes
buyer     seller    lender
==================================
'Smith'   'Jones'     'National Bank'
'Smith'   'Wilson'    'Home Bank'
'Nelson'  'Jones'     'Home Bank'

This table is a three-way relationship, but because many CASE tools allow only ...