database normalisation

database normalisation

(database)A series of steps followed to obtain a databasedesign that allows for efficient access and storage of datain a relational database. These steps reduce dataredundancy and the chances of data becoming inconsistent.

A table in a relational database is said to be in normalform if it satisfies certain constraints. Codd's originalwork defined three such forms but there are now five generallyaccepted steps of normalisation. The output of the first stepis called First Normal Form (1NF), the output of the secondstep is Second Normal Form (2NF), etc.

First Normal Form eliminates repeating groups by puttingeach value of a multi-valued attribute into a new row.

Second Normal Form eliminates functional dependencies on apartial key by putting the fields in a separate table fromthose that are dependent on the whole key.

Third Normal Form eliminates functional dependencies onnon-key fields by putting them in a separate table. At thisstage, all non-key fields are dependent on the key, the wholekey and nothing but the key.

Fourth Normal Form separates independent multi-valued factsstored in one table into separate tables.

Fifth Normal Form breaks out data redundancy that is notcovered by any of the previous normal forms.

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