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4NF ==>
database normalisation
<database> A series of steps followed to obtain a database
design that allows for efficient access and storage of data
in a relational database. These steps reduce data
redundancy and the chances of data becoming inconsistent.
A table in a relational database is said to be in normal
form if it satisfies certain constraints. Codd's original
work defined three such forms but there are now five generally
accepted steps of normalisation. The output of the first step
is called First Normal Form (1NF), the output of the second
step is Second Normal Form (2NF), etc.
First Normal Form eliminates repeating groups by putting
each into a separate table and connecting them with a
one-to-many relationship.
Second Normal Form eliminates functional dependencies on a
partial key by putting the fields in a separate table from
those that are dependent on the whole key.
Third Normal Form eliminates functional dependencies on
non-key fields by putting them in a separate table. At this
stage, all non-key fields are dependent on the key, the whole
key and nothing but the key.
Fourth Normal Form separates independent multi-valued facts
stored in one table into separate tables.
Fifth Normal Form breaks out data redundancy that is not
covered by any of the previous normal forms.
http://www.bkent.net/Doc/simple5.htm.
[What about non-relational databases?]
(2004-05-19)
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Nearby terms:
database machine « database management system « database manager « database normalisation » database query language » database server » Data/BASIC
This article was derived from the Free Online Dictionary of Computers and is available under ther terms of the GNU Free Documentation License.
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