- Fourth normal form
Fourth normal form (4NF) is a normal form used in
database normalization. Introduced by Ronald Faginin 1977, 4NF is the next level of normalization after Boyce-Codd normal form(BCNF). Whereas the second, third, and Boyce-Codd normal forms are concerned with functional dependencies, 4NF is concerned with a more general type of dependency known as a multivalued dependency. A table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies "X →→ Y", "X" is a superkey—that is, "X" is either a candidate keyor a superset thereof."A relation schema R* is in fourth normal form (4NF) if, whenever a nontrivial multivalued dependency X →→ 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|month=September|year=1977|pages=267|url=http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf|doi=10.1145/320557.320571]
If the column headings in a relational database table are divided into three disjoint groupings "X", "Y", and "Z", then, in the context of a particular row, we can refer to the data beneath each group of headings as "x", "y", and "z" respectively. A
multivalued dependency"X" →→ "Y" signifies that if we choose any "x" actually occurring in the table (call this choice "xc"), and compile a list of all the "xcyz" combinations that occur in the table, we will find that "xc" is associated with the same "y" entries regardless of z.
A trivial multivalued dependency "X" →→ "Y" is one in which "Y" consists of all columns not belonging to "X". That is, a subset of attributes in a table has a trivial multivalued dependency on the remaining subset of attributes.
functional dependencyis a special case of multivalued dependency. In a functional dependency "X" → "Y", every "x" determines "exactly one y", never more than one.
Consider the following example:
In contrast, if the pizza varieties offered by a restaurant sometimes did legitimately vary from one delivery area to another, the original three-column table would satisfy 4NF.
Ronald Fagin demonstratedFagin, 268] that it is always possible to achieve 4NF.
Rissanen's theoremis also applicable on multivalued dependencies.
4NF in practice
A 1992 paper by Margaret S. Wu notes that the teaching of database normalization typically stops short of 4NF, perhaps because of a belief that tables violating 4NF (but meeting all lower normal forms) are rarely encountered in business applications. This belief may not be accurate, however. Wu reports that in a study of forty organizational databases, over 20% contained one or more tables that violated 4NF while meeting all lower normal forms.cite journal|first=Margaret S.|last=Wu|title=The Practical Need for Fourth Normal Form|journal=ACM SIGCSE Bulletin|volume=24|issue=1|month=March|year=1992|pages=19–23|doi=10.1145/135250.134515]
Third normal form
Fifth normal form
* [http://www.datamodel.org/NormalizationRules.html Rules Of Data Normalization]
* Date, C. J. (1999), " [http://www.aw-bc.com/catalog/academic/product/0,1144,0321197844,00.html An Introduction to Database Systems] " (8th ed.). Addison-Wesley Longman. ISBN 0-321-19784-4.
* Kent, W. (1983) " [http://www.bkent.net/Doc/simple5.htm A Simple Guide to Five Normal Forms in Relational Database Theory] ", Communications of the ACM, vol. 26, pp. 120-125
* Date, C.J., & Darwen, H., & Pascal, F. " [http://www.dbdebunk.com Database Debunkings] "
* [http://www.utexas.edu/its/windows/database/datamodeling/rm/rm8.html Advanced Normalization] by ITS, University of Texas.
* [http://www.marcrettig.com/poster/ Free PDF poster available] by Marc Rettig
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