Editing Database normalization (section)

=== Satisfying ETNF ===
Suppose the franchisees can also order books from different suppliers. Let the relation also be subject to the following constraint:

* If a certain '''supplier''' supplies a certain '''title''' 
* and the '''title''' is supplied to the '''franchisee''' 
* and the '''franchisee''' is being supplied by the '''supplier,'''
* then the '''supplier''' supplies the '''title''' to the '''franchisee'''.<ref name=":2">{{Cite book|url=https://books.google.com/books?id=Jx5UCwAAQBAJ&q=etnf%20normalization&pg=PT138|title=The New Relational Database Dictionary: Terms, Concepts, and Examples|last=Date|first=C. J.|date=December 21, 2015|publisher="O'Reilly Media, Inc."|isbn=9781491951699|pages=138|language=en}}</ref>

{| class="wikitable"
|+Supplier - Book - Franchisee
!<u>Supplier ID</u>
!<u>Title</u>
!<u>Franchisee ID</u>
|-
|1
|Beginning MySQL Database Design and Optimization
|1
|-
|2
|The Relational Model for Database Management: Version 2
|2
|-
|3
|Learning SQL
|3
|}

This table is in [[Fourth normal form|4NF]], but the Supplier ID is equal to the join of its projections: '''<nowiki>{{Supplier ID, Title}, {Title, Franchisee ID}, {Franchisee ID, Supplier ID}}</nowiki>.''' No component of that join dependency is a [[superkey]] (the sole superkey being the entire heading), so the table does not satisfy the [[Essential tuple normal form|ETNF]] and can be further decomposed:<ref name=":2" />

{|
|
{| class="wikitable"
|+Supplier - Book
!<u>Supplier ID</u>
!<u>Title</u>
|-
|1
|Beginning MySQL Database Design and Optimization
|-
|2
|The Relational Model for Database Management: Version 2
|-
|3
|Learning SQL
|}
|
{| class="wikitable"
|+Book - Franchisee
!<u>Title</u>
!<u>Franchisee ID</u>
|-
|Beginning MySQL Database Design and Optimization
|1
|-
|The Relational Model for Database Management: Version 2
|2
|-
|Learning SQL
|3
|}
|
{| class="wikitable"
|+Franchisee - Supplier
!<u>Supplier ID</u>
!<u>Franchisee ID</u>
|-
|1
|1
|-
|2
|2
|-
|3
|3
|}
|}

The decomposition produces ETNF compliance.