Editing Fourth normal form (section)

{{short description|Normal form used in database normalization}}
'''Fourth normal form''' ('''4NF''') is a [[Database normalization#Normal forms|normal form]] used in [[database normalization]]. Introduced by [[Ronald Fagin]] in 1977, 4NF is the next level of normalization after [[Boyce–Codd normal form]] (BCNF). Whereas the [[Second normal form|second]], [[Third normal form|third]], and [[Boyce–Codd normal form|Boyce–Codd]] normal forms are concerned with [[functional dependency|functional dependencies]], 4NF is concerned with a more general type of dependency known as a [[multivalued dependency]]. A [[Table (database)|table]] is in 4NF [[if and only if]], for every one of its non-trivial multivalued dependencies ''X'' <math>\twoheadrightarrow</math> ''Y'', ''X'' is a [[superkey]]—that is, ''X'' is either a [[candidate key]] or a superset thereof.<ref name="Fagin">"A relation schema R* is in fourth normal form (4NF) if, whenever a nontrivial multivalued dependency X <math>\twoheadrightarrow</math> Y holds for R*, then so does the functional dependency X → A for every column name A of R*. Intuitively all dependencies are the result of keys." {{cite journal|first=Ronald|last=Fagin|title=Multivalued Dependencies and a New Normal Form for Relational Databases|journal=ACM Transactions on Database Systems|volume=2|issue=1|date=September 1977|pages=262–278|url=http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf|doi=10.1145/320557.320571|citeseerx=10.1.1.69.1872|s2cid=14617155 |access-date=2008-04-26|archive-url=https://web.archive.org/web/20071129092334/http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf|archive-date=2007-11-29|url-status=dead}}</ref>