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'''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=267|url=http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf|doi=10.1145/320557.320571}}</ref>
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==Multivalued dependencies==
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'' <math>\twoheadrightarrow</math> ''Y'' signifies that if we choose any ''x'' actually occurring in the table (call this choice ''x<sub>c</sub>''), and compile a list of all the ''x<sub>c</sub>yz'' combinations that occur in the table, we will find that ''x<sub>c</sub>'' is associated with the same ''y'' entries regardless of z.
 
A '''trivial multivalued dependency''' ''X'' <math>\twoheadrightarrow</math> ''Y'' is one where either ''Y'' is a subset of ''X'', or ''X'' and ''Y'' together form the whole set of attributes of the relation.
 
A [[functional dependency]] is a special case of multivalued dependency. In a functional dependency ''X'' → ''Y'', every ''x'' determines ''exactly one y'', never more than one.
 
==Example==
Consider the following example:
 
{| class="wikitable"
|+ Pizza Delivery Permutations
! <u>Restaurant</u> !! <u>Pizza Variety</u> !! <u>Delivery Area</u>
|-
|A1 Pizza||Thick Crust||Springfield
|-
|A1 Pizza||Thick Crust||Shelbyville
|-
|A1 Pizza||Thick Crust||Capital City
|-
|A1 Pizza||Stuffed Crust||Springfield
|-
|A1 Pizza||Stuffed Crust||Shelbyville
|-
|A1 Pizza||Stuffed Crust||Capital City
|-
|Elite Pizza||Thin Crust||Capital City
|-
|Elite Pizza||Stuffed Crust||Capital City
|-
|Vincenzo's Pizza||Thick Crust||Springfield
|-
|Vincenzo's Pizza||Thick Crust||Shelbyville
|-
|Vincenzo's Pizza||Thin Crust||Springfield
|-
|Vincenzo's Pizza||Thin Crust||Shelbyville
|}
 
Each row indicates that a given restaurant can deliver a given variety of pizza to a given area.
 
The table has no non-key attributes because its only key is {Restaurant, Pizza Variety, Delivery Area}. Therefore it meets all normal forms up to BCNF. If we assume, however, that pizza varieties offered by a restaurant are not affected by delivery area, then it does not meet 4NF. The problem is that the table features two non-trivial multivalued dependencies on the {Restaurant} attribute (which is not a superkey). The dependencies are:
 
* {Restaurant} <math>\twoheadrightarrow</math> {Pizza Variety}
* {Restaurant} <math>\twoheadrightarrow</math> {Delivery Area}
 
These non-trivial multivalued dependencies on a non-superkey reflect the fact that the varieties of pizza a restaurant offers are independent from the areas to which the restaurant delivers. This state of affairs leads to [[Data redundancy|redundancy]] in the table: for example, we are told three times that A1 Pizza offers Stuffed Crust, and if A1 Pizza starts producing Cheese Crust pizzas then we will need to add multiple rows, one for each of A1 Pizza's delivery areas. There is, moreover, nothing to prevent us from doing this incorrectly: we might add Cheese Crust rows for all but one of A1 Pizza's delivery areas, thereby failing to respect the multivalued dependency {Restaurant} <math>\twoheadrightarrow</math> {Pizza Variety}.
 
To eliminate the possibility of these anomalies, we must place the facts about varieties offered into a different table from the facts about delivery areas, yielding two tables that are both in 4NF:
 
{| class="wikitable" style="margin: 10px; float:left;"
|+ Varieties By Restaurant
! <u>Restaurant</u> !! <u>Pizza Variety</u>
|-
|A1 Pizza||Thick Crust
|-
|A1 Pizza||Stuffed Crust
|-
|Elite Pizza||Thin Crust
|-
|Elite Pizza||Stuffed Crust
|-
|Vincenzo's Pizza||Thick Crust
|-
|Vincenzo's Pizza||Thin Crust
|}
{| class="wikitable" style="margin: 10px; float:left;"
|+ Delivery Areas By Restaurant
! <u>Restaurant</u> !! <u>Delivery Area</u>
|-
|A1 Pizza||Springfield
|-
|A1 Pizza||Shelbyville
|-
|A1 Pizza||Capital City
|-
|Elite Pizza||Capital City
|-
|Vincenzo's Pizza||Springfield
|-
|Vincenzo's Pizza||Shelbyville
|}
{{-}}
 
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 demonstrated that it is always possible to achieve 4NF.<ref name="Fagin268">Fagin, p. 268</ref> [[Rissanen's theorem]] is also applicable on [[Multivalued dependency|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.<ref name="Wu">{{cite journal|first=Margaret S.|last=Wu|title=The Practical Need for Fourth Normal Form|journal=ACM SIGCSE Bulletin|volume=24|issue=1|date=March 1992|pages=19–23|doi=10.1145/135250.134515}}</ref>
 
==See also==
*[[Attribute-value system]]
<!-- See navigation box at bottom of page. -->
 
==References==
{{Reflist}}
 
==Further reading==
{{Refbegin}}
*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.&nbsp;120–125
*[http://www.utexas.edu/its/windows/database/datamodeling/rm/rm8.html Advanced Normalization] by ITS, University of Texas.
{{Refend}}
 
{{Database normalization}}
 
[[Category:Database normalization|4NF]]
 
[[de:Normalisierung (Datenbank)#Vierte Normalform (4NF)]]

Latest revision as of 12:02, 27 March 2014

We can discover easy techniques to speed up computer by making the most from the built in tools in a Windows and also getting the Service Pack updates-speed up your PC plus fix error. Simply follow a limited policies to swiftly make your computer swiftly than ever.

StreamCI.dll mistakes are caused by a amount of different problems, including that the file itself has been moved on a program, the file is outdated or you have installed certain third-party audio motorists that are conflicting with all the file. The wise news is that if you would like to solve the error you're seeing, you need to look to initially guarantee the file & motorists are working okay on the PC and also then resolving any StreamCI.dll errors which may be inside the registry of the computer.

The Windows registry is a system database of information. Windows plus different software store a lot of settings and different info inside it, plus retrieve such info within the registry all time. The registry is moreover a bottleneck in which considering it happens to be the heart of the running program, any problems with it can result mistakes and bring the running program down.

Fixing tcpip.sys blue screen is easy to do with registry repair software.Trying to fix windows blue screen error on your could be tricky considering if you remove or damage the registry it may cause severe damage to your computer. The registry needs to be cleaned and all erroneous plus incomplete information removed to stop blue screen errors from occurring.The benefit of registry repair software is not limited to just getting rid of the blue screen on business.You may be surprised at the greater plus more improved speed and performance of the computer program following registry cleaning is performed. Registry cleaning can definitely develop the computer's functioning abilities, incredibly when we choose a certain registry repair software which is extremely powerful.

Many iolo system mechanic s allow we to download their product for free, so you can scan the computer oneself. That way you can see how several mistakes it finds, where it finds them, plus how it may fix them. A amazing registry cleaner may remove your registry issues, plus optimize and accelerate your PC, with small effort on a piece.

Software errors or hardware mistakes that happen whenever running Windows plus intermittent errors are the general reasons for a blue screen physical memory dump. New software or drivers which have been installed or changes in the registry settings are the typical s/w causes. Intermittent errors refer to failed program memory/ difficult disk or over heated processor and these too could result the blue screen physical memory dump error.

The System File Checker (SFC) can aid inside resolving error 1721 as it, by its nature, scans the program files for corruption and replaces them with their authentic versions. This requires you to have the Windows Installation DVD ROM for continuing.

So inside summary, whenever comparing registry cleaning, ensure that the 1 we choose provides you the following.A backup and restore facility, quick operation, automatic deletion center, start-up administration, an simple technique of contact and a funds back guarantee.

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