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.

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[What about non-relational databases?]

(1999-05-05)